Astronomy

What are the differences between matter, dark matter and antimatter?

What are the differences between matter, dark matter and antimatter?

I thought dark and anti matter were kinda the same, but after saw a video, they mention that dark matter is not antimatter but their explanation is a little fast so I got doubts.

What are the differences between matter, dark matter and antimatter? Are they related? How they interact each other? Where can I see an example of this interaction? and also, in terms of percentage, how much of matter, dark matter and antimatter exist in the universe?

Thanks!


Matter is the stuff you are made of.

Antimatter is the same as matter in every way, looks the same, behaves the same, except its particles have electrical charges opposite to matter. E.g., our electrons are negatively charged, whereas a positron (an antimatter "electron") is positively charged. The positron is the "anti-particle" of the electron.

When a particle meets its anti-particle, they "annihilate": the two particles disappear, and gamma photons are released carrying off their energy. For this reason, should a lump of matter touch a lump of antimatter, they would annihilate, and a giant explosion would result because of the huge energy released (E=mc^2).

Matter and antimatter are definitely related: same thing, but with opposite signs. Twins, but opposites.

It is not clear why, but it seems like there isn't that much antimatter out there, more like trace amounts. Definitely not as much as regular matter as far as we can tell. This is puzzling to physicists and cosmologists, because you'd expect the Big Bang to make roughly equal amounts of matter and antimatter. Scientists agree that the paradox of "excess matter" will advance physics even further once it's solved.


Dark matter - we don't really know what it is. It's not even sure it's "matter" in a conventional sense, or related to it in any way. We just know that galaxies are rotating in such a way that indicates there's a lot more mass out there, but it is mass that we cannot see and cannot be accounted for in the usual ways. Hence the name "dark" (as in invisible) matter.

Dark matter doesn't seem to interact much with regular matter, except gravitationally. Right now dark matter could be passing through you and you wouldn't notice. Dark matter also does not interact with light, so you can't see it. It doesn't seem to interact much with itself either, so for this reason dark matter cannot form "clumps" such as planets or stars. Instead, it probably exists in a diffuse form. Bottom line, dark matter interacts pretty much only via gravity.

The shape of galaxies is a proof of the existence of dark matter, and is a result of the interaction between matter and dark matter. Without dark matter, galaxies would be much less massive, and the outer parts would rotate much more slowly compared to the center. Due to dark matter, galaxies are quite massive, and they rotate almost as solid objects - the outer parts rotate approximately as fast as the central parts.

Estimates vary, but it seems like there's something like 5x to 6x more dark matter out there compared to regular matter.


A pretty good site for quick explanations is the Particle Adventure

Anti Matter

Dark Matter

Anti Matter is really quite simple and very similar to regular matter. It just happens to explode violently when it touches regular matter - like a positron (Positive Electron) and an electron (negative) will touch and evaporate into a pair of gamma rays. The Proton/anti Proton or Neutron/anti Neutron or Proton/antineutron or Neutron/antiProton (they interact because they have some quarks/anti-quarks in common). Those reactions are more complicated, but the gist is the same. They explode violently when they touch. So, because matter and anti matter tend to evaporate each other, there's really no primordial anti-matter left in the universe, cause there was slightly more matter.

But, other than the explosive interaction, Antimatter is almost exactly the same as matter, You could, in theory build a star, a planet, trees and life out of anti-matter.

Dark Matter is a lot more different. We don't really know what it is, but it's a different kind of matter, it's transparent and it's non binding but it has mass. Regular matter can bind together gravitationally, that's why it forms into things like stars, planets, comets, asteroids, etc. Dark matter doesn't do that. It loosely collects around galaxies, or, perhaps more accurately, galaxies collect inside large pockets of dark matter.


The Difference between Dark Matter and Dark Energy

Our universe is expanding more than ever, since its origin from the Big Bang, 14 billion years ago. Previously, the scientists thought it would only slow down because of the gravitational pull that attracts all matter towards the inside. But, the Hubble Space Telescope observations prove that the Universe is actually expanding rather than slowing down. This cannot happen without the presence of some other form of energy that is superior to the gravitational strength, though no one knows what it is. This incomprehensible energy, which repels matter outward, is called Dark energy. The visible matter, including the Earth, stars and billions of galaxies, made of subatomic particles clustered into atoms, constitute only 4% of the mass of the universe. We do not know the content of the other mass, except that 22% of it is the invisible substance called Dark matter , and 74% is the ever dominating Dark energy. Though both can be measured by calculating their effect on the detectable matter of the universe, it is not known whether these two are one and the same.

Dark energy is omnipresent and its effect increases as cosmos swells. Its existence enables light to gain energy from the residual radiation if it travels through large masses, and is responsible for cosmic microwaves. When gravity becomes weaker due to the space expansion, dark energy will begin to dominate. It is assumed that it is this dark energy that is responsible for the expansion of the universe. Dark energy, which also is known as cosmological constant as well as quintessence, accelerates the expansion process by becoming an anti-gravity force. According to Albert Einstein, the empty space is seldom vacuum and has own constant energy to force the universe expand faster and faster.

(A 2009 simulation of dark matter in the Universe)

Opposing Einstein’s observation, new theories have evolved explaining dark energy as a new form of dynamical energy fluid that fills the space, which works against matter and normal energy. Some researchers find quantum fluctuations as the real source of the repulsive force accelerating the space expansion. However, all agrees that Dark energy, being uniform throughout the space, is behind the faster rate of acceleration of the expanding cosmos, though its density is low (6.91 × 10−27 kg/m3) compared to the density of ordinary matter or the dark matter of the galaxies. In spite of all of these observations, sceptics emphasize that it is nothing but an illusion caused by the relative movement of the Earth with the rest of the cosmos. Whatever it is, Dark energy is the greatest scientific mystery of our time.

Dark matter is non-luminous particles of matter that exerts gravitational effects on the visible matter of galaxies and clusters of galaxies. It is dark, invisible, and covers most of the cosmic matter. Scientists could not observe it directly as it is not possible to detect it with what they have as instruments, today. But its presence is unequivocally confirmed by its gravitational effects. It is this gravity of dark matter that pulls the universe together, keeping it away from collapse. If the universe contains only detectable matter, the galaxies we see would not have emerged at all. They would only fly apart without having enough matter of gravitational force to keep them close together. In the beginning of the universe, the dominating Dark matter amplified low fluctuations in the Cosmic Microwave background to make the present universe.

As per astrophysics, dark matter is undetectable, non-baryonic matter which exerts gravitational effects on stars and galaxies. It is a hypothetical particle without any charge, no spin, and insignificant mass composed by quantum chromo dynamics. Also, there are chances that it might be formed out of exotic particles like axions or weakly interacting massive particles, immediately after the creation of the universe. It is interesting to note that t he existence of dark matter was discovered accidentally while observing the outer regions of the Milky Way. I f the efforts of scientists to recognize dark matter continue without any fulfilment in the near, such an improbability poses a question: What if the universe ends, all of a sudden?


Matter vs. Antimatter

Fictional starships notwithstanding, there’s not much antimatter in the universe. And for us, that’s a good thing. Any time matter and antimatter meet, they cancel each other out in a blaze of energy.

Antimatter is identical to normal matter in almost every way. The only difference is electric charge, which is opposite for the two forms of matter. So there could be a whole galaxy made of antimatter out there and our telescopes wouldn’t see it any differently from a galaxy of normal matter.

Most theories say the Big Bang should have created equal amounts of matter and antimatter. But in the first tiny fraction of a second, something changed that balance. For every billion pairs of matter and antimatter particles, there was one extra particle of matter.

One of the first scientists to consider that imbalance was Andrei Sakharov. The Russian physicist had helped develop the Soviet hydrogen bomb, but turned away from weapons work. In a paper published 50 years ago, he outlined conditions that could create the imbalance.

Sakharov said that protons must decay, but so slowly that it’s almost impossible to detect. Second, he said that the universe must have cooled in a certain way in the moments after the Big Bang. And finally, he said there must be some difference between matter and antimatter.

So far, none of those conditions has been found to account for the imbalance between matter and antimatter, so the subject remains a busy topic of research.

Script by Damond Benningfield


What Happened to All of the Universe&rsquos Antimatter?

We could have been living in an antimatter universe, but we are not. Antimatter is matter&rsquos upside-down twin&mdashevery matter particle has a matching antimatter version with the opposite charge. Physicists think the cosmos started out with just as much antimatter as matter, but most of the former got wiped out. Now they may be one step closer to knowing why.

Researchers at the Large Hadron Collider Beauty (LHCb) experiment at CERN near Geneva have discovered antimatter and matter versions of &ldquocharm&rdquo quarks&mdashone of six types, or flavors, of a class of elementary matter particles&mdashacting differently from one another. In a new study, which was presented in March at the &ldquoRencontres de Moriond&rdquo particle physics conference in La Thuile, Italy, the physicists found that unstable particles called D 0 mesons (which contain charm quarks) decayed into more stable particles at a slightly different rate than their antimatter counterparts. Such differences could help explain how an asymmetry arose between matter and antimatter after the big bang, resulting in a universe composed mostly of matter.

Matter and antimatter annihilate each other on contact, and researchers believe such collisions destroyed almost all of the antimatter (and a large chunk of the matter) that initially existed in the cosmos. But they do not understand why a relatively small excess of matter survived to become the stars and planets and the rest of the cosmos. Consequently, physicists have been looking for a kind of matter that behaves so differently from its antimatter version that it would have had time to generate this excess in the early universe.

The newly discovered mismatch in decay rates between charm quarks and antiquarks turns out to be too small to account for the universe&rsquos excess of matter. The result, however, &ldquodoes bring us closer to finding the answer because it shows one of the possible answers may not be the right one,&rdquo says theoretical physicist Yuval Grossman of Cornell University, who was not involved in the new work. &ldquoI am also excited because it&rsquos the first time we&rsquove ever seen this [phenomenon in charm quarks].&rdquo

Physicists previously found similar variations in two other quark flavors, but those were also too tiny to account for our matter-dominated universe. Scientists are holding out hope of finding much larger matter-antimatter differences elsewhere, such as in ghostly particles called neutrinos or reactions involving the Higgs boson&mdashthe particle that gives others mass&mdashsays LHCb team member Sheldon Stone of Syracuse University: &ldquoThere are lots of different searches going on.&rdquo

ABOUT THE AUTHOR(S)

Clara Moskowitzis Scientific American's senior editor covering space and physics. She has a bachelor's degree in astronomy and physics from Wesleyan University and a graduate degree in science journalism from the University of California, Santa Cruz.


What happens when Matter and Antimatter Meet

While most people have some idea what matter is, the may not have any idea what antimatter is. Antimatter may sound like science fiction, perhaps because the word has been used frequently in science fiction, but it is in fact real. Interestingly enough, matter and antimatter are actually very similar to each other except for one single difference, but what happens when they meet?

Everything that is around us consists of matter. Matter is basically anything that occupies space and has mass. Matter is composed of molecules, which are composed of atoms, which in turn have particles such as protons, neutrons, and electrons. These are just a few of the fundamental particles which are the basic units of matter. There are twelve different types of fundamental particles in total.

Antimatter is similar to matter in every way. It occupies space, has mass, and is made up of fundamental particles. However, antimatter has an opposing charge. This is the only difference between matter and antimatter. For every fundamental particle, there is an antimatter equivalent particle. For example, whereas matter has protons, antimatter has antiprotons. As a result of the opposite charge, antimatter also has opposing magnetic properties.

In terms of the amount of antimatter in the universe, it really only exists as a byproduct of a high energy events. This small amount of antimatter is also quickly annihilated. Scientists are not sure why so much more matter exists than antimatter. It is possible that an appreciable amount of antimatter just hasn&rsquot been found yet.

What happens when they meet?

Matter and Antimatter meet in a process called annihilation. Both particles will disappear after meeting. The mass of both particles is instantly converted to energy in this reaction. The resultant energy is usually a photon, which is a single particle of light. The process of annihilation is also reversible. In the presence of a large enough amount of energy, both matter and antimatter can be created. This is the part of the Big Bang theory. A large enough amount of energy produced the matter that formed the universe. All of the antimatter produced during the Big Bang was annihilated in the presence of the matter. However, this does not explain why the universe has such a large amount of matter and why everything that we know of it is created from matter and not antimatter. Theoretically, it was possible that everything familiar could have been created out of antimatter instead and scientists are not sure why matter makes up the galaxy instead of antimatter.


Link between antimatter and dark matter probed

Gianpaolo Carosi is in the Rare Event Detection Group, Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, USA.

You can also search for this author in PubMed Google Scholar

Two of the most intriguing mysteries in modern cosmology are the apparent preponderance of ordinary matter over antimatter and the nature of dark matter, which accounts for about 85% of the mass in the Universe 1 . Dark matter has made its presence known only through its gravitational effects on astrophysical objects. Therefore, whatever type of particle it is made of must have feeble interactions with other matter. One leading candidate is the axion — a light neutral particle that was originally postulated to explain why the neutron lacks a measurable electric dipole moment 2 . Until now, researchers have looked for evidence of couplings between axion dark matter and only ordinary particles such as photons, electrons and nuclei 3 , 4 . Writing in Nature, Smorra et al. 5 report a search for a coupling between axion dark matter and antimatter (specifically, antiprotons).

Read the paper: Direct limits on the interaction of antiprotons with axion-like dark matter

Every known particle can be classified as either a boson or a fermion. Bosons have integer spin (intrinsic angular momentum), and include the (spin-1) photon and the (spin-0) Higgs boson. By contrast, fermions have half-integer spin, and include the (spin-1/2) electron. The axion is expected to be a spin-0 boson that has odd parity, which means that its wavefunction changes sign if spatial coordinates are flipped.

Unlike fermionic dark matter (such as dark-matter candidates called weakly interacting massive particles, WIMPs), there is no limit to the number of axions that can exist in a certain volume of space. As a result, axion dark matter has an extremely wide range of potential masses. Astrophysical measurements place an upper limit 6 on the mass of about 10 –2 electronvolts (eV). This value is expressed in units of energy, in which the electron mass is 511 kiloelectronvolts and the proton mass is 938 megaelectronvolts (see go.nature.com/2bwkrqz). And a lower limit 7 of about 10 –22 eV comes from the fact that, when these particles are described as waves in quantum mechanics, their wavelengths cannot be larger than the size of a dwarf galaxy — otherwise, such galaxies would show deviations from their observed structure.

The particles associated with axion dark matter can be thought of as classical waves that have an oscillation frequency directly proportional to the axion mass. There are several techniques that can be used to look for such waves, and the most appropriate one depends mainly on the frequency range that is being considered. For axions that have masses below 10 –17 eV (corresponding to a frequency of tens of millihertz), the waves oscillate extremely slowly. If antiprotons are held in the strong magnetic field of a device known as a Penning trap, these waves will produce changes in the frequency at which the spins of the antiprotons precess.

Dark-matter detector observes exotic nuclear decay

The Baryon Antibaryon Symmetry Experiment 8 (BASE) at the European particle-physics laboratory CERN near Geneva, Switzerland, uses this technique. The BASE collaboration relies on ultrasensitive Penning traps, which use specialized configurations of magnetic and electric fields to trap antiprotons in a high-vacuum environment. This set-up allows the antiprotons to be measured continuously for long periods of time, and to be shuttled back and forth between different measurement chambers without running into ordinary matter and being annihilated. One of the main goals of the collaboration is to determine the intrinsic magnetic moment of the antiproton. This quantity can be calculated to extremely high precision using the standard model of particle physics — the current explanation of the Universe’s particles and forces.

In 2017, Smorra et al. made an ultraprecise measurement of the antiproton’s magnetic moment (to one part in a billion) 9 , constraining many theories of physics beyond the standard model. The key to their method was the simultaneous measurement of the spin precession and a quantity called the cyclotron frequency, which describes the cyclical motion of an antiproton in a trap. This task was challenging, because it required meticulous control of a device known as a magnetic bottle to non-destructively determine the spin state of the antiproton. The group’s measurement required hundreds of experiments, each of which lasted for almost an hour, taking place over several months.

In the current paper, Smorra and colleagues, who include members of the BASE collaboration, analysed the data from these experiments. They proposed that waves corresponding to axion dark matter that oscillated at frequencies between 10 –8 and 10 –2 hertz would shift the spin-precession frequency in a small but measurable way if the axion coupling to antiprotons was sufficiently strong. Although no axion signal was detected, Smorra et al. constrained the parameter that quantifies axion–antiproton interactions to values greater than 0.1–0.6 gigaelectronvolts in the axion mass range from 2 × 10 −23 eV to 4 × 10 −17 eV (Fig. 1). These limits are as much as 10 5 times stronger than astrophysical constraints (as estimated by the authors), which consider how axions might have been produced by antiprotons in the supernova 1987A.

Figure 1 | Constraining axion–antiproton interactions. Particles called axions could account for the elusive dark matter that pervades the Universe. Smorra et al. 5 present experimental limits on the coupling between axion dark matter and antiprotons. These bounds are expressed in terms of an axion–antiproton interaction parameter and vary with the axion mass or the frequency of the axion if the particle is represented as a wave (eV, electronvolts GeV, gigaelectronvolts Hz, hertz). The combined limit represents the strongest constraint that could be set by the experimental data. An astrophysical limit, as estimated by the authors, is included for comparison. The coloured and hatched areas show the parameter space that is excluded.

Future work should aim to further constrain the axion–antiproton coupling and to look for evidence of interactions between axion dark matter and other forms of antimatter, such as positrons (the antiparticles of electrons). One key finding from these studies could be the observation that dark matter couples to antimatter in different ways from its couplings to ordinary matter — a result that might help to explain why there is a predominance of matter over antimatter in the Universe.

Smorra and colleagues have highlighted a growing trend in high-energy physics, whereby exquisitely precise measurements are used to nail down fundamental particle parameters and to look for evidence of physics beyond that of the standard model. Axion dark matter, which has a vast potential mass range and extraordinarily weak predicted couplings, has gone through a renaissance in terms of innovative detection techniques. The search for a preferred coupling of axion dark matter to antimatter (as opposed to ordinary matter) is an exciting prospect, and could prove to be the key to unlocking several mysteries in cosmology as technology improves.


New Map of Local Dark Matter Reveals ‘Bridges’ between Galaxies

A team of astrophysicists from the United States and Korea has created a new dark-matter distribution map using a neural network-based deep learning method and the data on positions and velocities of galaxies in the local Universe.

The 3D density map of the local dark matter: X-mark at the center denotes the Milky Way Galaxy dots denote galaxies, and arrows denote estimated directions of motion derived from the gradient of the reconstructed gravitational potential. Image credit: Hong et al., doi: 10.3847/1538-4357/abf040.

“The 80% of the matter in the Universe is in the form of dark matter that comprises the skeleton of the large-scale structure called the cosmic web,” said Dr. Donghui Jeong, an astrophysicist in the Department of Astronomy and Astrophysics and the Institute for Gravitation and the Cosmos at the Pennsylvania State University.

“As the cosmic web dictates the motion of all matter in galaxies and inter-galactic media through gravity, knowing the distribution of dark matter is essential for studying the large-scale structure.”

“However, the cosmic web’s detailed structure is unknown because it is dominated by dark matter and warm-hot inter-galactic media, both of which are hard to trace.”

In the study, Dr. Jeong and colleagues took a completely different approach, using machine learning to build a model that uses information about the distribution and motion of galaxies to predict the distribution of dark matter.

They built and trained their model using a large set of galaxy simulations, called Illustris-TNG, which includes galaxies, gasses, other visible matter, as well as dark matter.

They specifically selected simulated galaxies comparable to those in the Milky Way and ultimately identified which properties of galaxies are needed to predict the dark matter distribution.

“When given certain information, the model can essentially fill in the gaps based on what it has looked at before,” Dr. Jeong said.

“The map from our models doesn’t perfectly fit the simulation data, but we can still reconstruct very detailed structures.”

“We found that including the motion of galaxies — their radial peculiar velocities — in addition to their distribution drastically enhanced the quality of the map and allowed us to see these details.”

The researchers then applied their model to real data from the local Universe from the Cosmicflow-3 galaxy catalog.

The map successively reproduced known prominent structures in the local Universe, including the Local Sheet (a region of space containing the Milky Way, nearby galaxies in the Local Group, and galaxies in the Virgo Cluster) and the Local Void (a relatively empty region of space next to the Local Group).

Additionally, it identified several new structures that require further investigation, including smaller filamentary structures that connect galaxies.

“Having a local map of the cosmic web opens up a new chapter of cosmological study,” Dr. Jeong said.

“We can study how the distribution of dark matter relates to other emission data, which will help us understand the nature of dark matter.”

“And we can study these filamentary structures directly, these hidden bridges between galaxies.”

Sungwook E. Hong et al. 2021. Revealing the Local Cosmic Web from Galaxies by Deep Learning. ApJ, in press doi: 10.3847/1538-4357/abf040


Charming discovery

Physicists have long known that certain interactions between particles create differences in the behaviours of matter particles and their antimatter counterparts. This phenomenon, which creates the matter–antimatter imbalance, is known as CP violation.

Since the 1960s, physicists have found CP violation in particles called kaons and in B mesons, which are each made up of two quark particles — observations that contributed to work that won the Nobel Prizes in Physics in 1980 and 2008.

But, until now, CP violation had’nt been seen in a particle that includes the ‘charm’ flavour of quark, such as a D meson. “Observing that matter and antimatter charm mesons behave differently provides a measurement for the textbooks,” says Tara Shears, a particle physicist at the University of Liverpool, UK, and a member of the LHCb team.

Physicists know, however, that the dominance of matter can’t be explained by the behaviour of quarks and antiquarks alone, and finding new kinds of CP violation remains one of the biggest challenges of particle physics.

The effect in D mesons is so small that it is technically extremely difficult to measure, says Shears. It took from 2011 to 2018 to accumulate enough particle decays for the data set to be sensitive to the slight imbalance.

“It’s really a testament to the fantastic precision and sensitivity of the LHCb experiment, the ingenuity of the physicists analysing the data, and the ability of LHC to deliver huge samples, that this is now possible,” Shears says.


What are the differences between matter, dark matter and antimatter? - Astronomy

I've been reading many clever answers here about dark matter and dark energy that called my attention to this question. Since Einstein's theory relates matter and energy as different states of the same thing, is it valid to think about dark matter and dark energy in the same way? Are they two states of the same dark "thing"? Are they interchangeable?

The short answer to your question is that we don't know if dark matter and dark energy are manifestations of the same dark "thing". We know they both must exist to explain certain phenomena, but we still know very little about their make up so we cannot assume they are linked. For now, we think of them as separate, and we believe the cosmos to be composed of roughly 0.03% heavy elements (anything other than hydrogen and helium), 0.3% neutrinos, 0.5% stars, 4% free hydrogen and helium, 25% dark matter, and 70% dark energy. Here is how we define them separately:

Dark matter must exist to account for the gravity that holds galaxies together. If the only matter in the universe was matter we could directly detect, galaxies would not have had enough matter to have ever formed. The galaxies we observe today would fly apart because they wouldn't have enough matter to create a strong enough gravitational force to hold themselves together. Dark matter is also responsible for amplifying small fluctuations in the Cosmic Microwave Background back in the early universe to create the large scale structure we observe in the universe today.

Dark energy, which also goes by the names of the cosmological constant or quintessence, must exist due to the rate of expansion we observe for our universe. Not only is the universe expanding, but this expansion is also accelerating so the unknown 'anti-gravity' force at work is termed 'dark energy'.

Some researchers are searching for an explanation that encompasses both dark matter and dark energy. One example of such a theory uses a form of energy called a scalar field (it is a field because it has magnitude, energy and pressure, but it is scalar so it has no direction). Things would certainly be easier if we didn't need to have separate theories to explain dark matter and dark energy. However, other researchers look at dark matter and dark energy as two separate problems. For example, many string theories use supersymmetric particles to explain dark matter and make no connection to dark energy at all.

This page was last updated June 27, 2015.

About the Author

Sabrina Stierwalt

Sabrina was a graduate student at Cornell until 2009 when she moved to Los Angeles to become a researcher at Caltech. She now studies galaxy mergers at the University of Virginia and the National Radio Astronomy Observatory in Charlottesville. You can also find her answering science questions in her weekly podcast as Everyday Einstein.


What are the differences between matter, dark matter and antimatter? - Astronomy

I've been reading many clever answers here about dark matter and dark energy that called my attention to this question. Since Einstein's theory relates matter and energy as different states of the same thing, is it valid to think about dark matter and dark energy in the same way? Are they two states of the same dark "thing"? Are they interchangeable?

The short answer to your question is that we don't know if dark matter and dark energy are manifestations of the same dark "thing". We know they both must exist to explain certain phenomena, but we still know very little about their make up so we cannot assume they are linked. For now, we think of them as separate, and we believe the cosmos to be composed of roughly 0.03% heavy elements (anything other than hydrogen and helium), 0.3% neutrinos, 0.5% stars, 4% free hydrogen and helium, 25% dark matter, and 70% dark energy. Here is how we define them separately:

Dark matter must exist to account for the gravity that holds galaxies together. If the only matter in the universe was matter we could directly detect, galaxies would not have had enough matter to have ever formed. The galaxies we observe today would fly apart because they wouldn't have enough matter to create a strong enough gravitational force to hold themselves together. Dark matter is also responsible for amplifying small fluctuations in the Cosmic Microwave Background back in the early universe to create the large scale structure we observe in the universe today.

Dark energy, which also goes by the names of the cosmological constant or quintessence, must exist due to the rate of expansion we observe for our universe. Not only is the universe expanding, but this expansion is also accelerating so the unknown 'anti-gravity' force at work is termed 'dark energy'.

Some researchers are searching for an explanation that encompasses both dark matter and dark energy. One example of such a theory uses a form of energy called a scalar field (it is a field because it has magnitude, energy and pressure, but it is scalar so it has no direction). Things would certainly be easier if we didn't need to have separate theories to explain dark matter and dark energy. However, other researchers look at dark matter and dark energy as two separate problems. For example, many string theories use supersymmetric particles to explain dark matter and make no connection to dark energy at all.

This page was last updated June 27, 2015.

About the Author

Sabrina Stierwalt

Sabrina was a graduate student at Cornell until 2009 when she moved to Los Angeles to become a researcher at Caltech. She now studies galaxy mergers at the University of Virginia and the National Radio Astronomy Observatory in Charlottesville. You can also find her answering science questions in her weekly podcast as Everyday Einstein.