Astronomers know that dark matter and energy exist; however, they're still uncertain what they consist of.
Dark matter, also known as cosmic dark energy (CDM), influences the movements of stars and gas within galaxies. Since it doesn't emit or absorb light directly, dark matter can be difficult to detect directly.
At present, it is thought to consist of non-baryonic matter; leading candidates for this include WIMPS (weakly interacting massive particles), neutrinos, and axions.
What We Know
Dark matter, an invisible mass held together by gravity, was first identified through observations made in the 1930s by Fritz Zwicky and later verified by Vera Rubin. Astrophysicists know dark matter exists through its gravitational effects: rotation speeds of individual galaxies, distribution of galaxies within galaxy clusters and cosmic microwave background anisotropy anisotropies are indicators. Furthermore, dark matter influences galaxies through gravitational lensing or mass position changes during galactic collisions.
Physical scientists know one key thing about dark matter: It does not interact with electromagnetic force, making it nearly invisible. Scientists only know it exists because its gravitational effect outweighs regular matter by about six to one; visible matter makes up only 27% of total universe matter. Since the 1990s, scientists have constructed and operated large experiments designed to detect dark matter; so far however they have come up empty-handed.
To counter that issue, physicists have devised various theories to explain the nature of dark matter. Some theories propose that dark matter may contain supersymmetric particles - hypothetical particles that act as counterparts to those already found within the Standard Model of particle physics.
If dark matter consists of supersymmetric particles, then its constituent particles should have an extremely low energy density compared to ordinary matter, meaning that its particles could move very rapidly without losing much energy in their motions and creating new matter at smaller scales - possibly explaining why its distribution across space seems the way it does.
Dark matter could also exist without being created this way; that would indicate that Albert Einstein's theory of general relativity may not be fully accurate. Euclid telescope has already begun mapping galaxies 10 billion years back. Scientists will need to assess this data carefully in order to make this determination.
What We Don?t Know
Physical scientists employ several strategies for searching for dark matter. First, they can search for particles that interact only through gravity with ordinary matter; one way of doing this involves searching for their signature tracks left by neutron stars or supernova explosion debris; or by measuring their gravitational influence through gravitational lensing of images produced by galaxies and quasars.
Scientists can also search for dark matter particles in laboratories. One such experiment is the Large Underground Xenon (LUX) experiment conducted at Sanford Underground Research Facility in South Dakota, using huge tanks filled with super-chilled liquid xenon deep below Earth. While the experiment failed to find dark matter particles, particle theorists celebrated it with the following joke: if "if you have to have nothing, then make it good nothing!"
Astronomers can search for dark matter by measuring fluctuations in the cosmic microwave background (CMB), caused by its scattering by ordinary matter. Astronomers can also observe galaxies moving across space by monitoring sound waves produced from neutron-proton collisions within stars, or track cosmic redshift (variations in speed of light travel through it) through measurements.
These measurements often suggest there is more matter out there than we know about, indicating unknown matter is present. Such evidence includes rotational velocity of stars and their speeds of travel as well as radiation emissions that correlate to Einstein's famous equation of E = mc2.
One theory about dark matter includes the possibility that it consists of supersymmetric particles - hypothetical counterparts to those present in the Standard Model of particle physics - though no such particles have yet been identified and searched for by researchers.
What We Can Do
Scientists have deduced the existence of dark matter and dark energy through studying their cosmic effects, but haven't managed to directly detect either yet. Multiple Fermilab experiments are working towards changing that.
Astronomers first recognized the presence of dark matter in 1933 when they observed that galaxies' rapid movements within clusters could not be explained by visible mass alone. Astronomers eventually came to believe that some unseen mass exerted its gravitational pull on these galaxies and clusters; it left no visible traces in the cosmic microwave background (CMB), creating excess gamma-ray emissions at galaxies' centers; though these indicators were insufficient evidence of dark matter's existence.
One of the most promising techniques for future exploration is called weak gravitational lensing, which uses Einstein's theory of gravity to locate particles of dark matter. When two galaxy clusters lie along an intersecting path of sight, when one acts as a lens it bends light coming from its background cluster to provide scientists with insight into how much dark matter might exist in its foreground clusters - thus giving scientists insight into its concentration there.
Searches for dark matter can also take the form of large-scale surveys such as those being undertaken by DESI - Dark Energy Survey International - an ongoing project jointly led by Fermilab which will capture images from 30 million galaxies over five years, giving its team access to immense amounts of data that provides clues as to where evidence of dark matter and dark energy exists.
Another avenue is studying superclusters, or groups of galaxies clumped together, by astronomers. Astronomers aim to understand what causes superclusters and their interactions with dark matter and dark energy; but this requires studying extremely faint signals from individual particles within the cosmic microwave background that even large telescopes cannot pick up on.
What We Can?t Do
Prior to recently, all we could do to estimate how much dark matter exists in the universe was estimate its presence through observations of distant galaxies like our Milky Way and Andromeda that moved quickly through space. From these movements came evidence of large-scale webs of dark matter pulling galaxies closer together while dark energy pulled them apart; yet these measurements proved more complicated due to galaxies not just being far away but also moving rapidly, potentially altering results and producing misleading measurements.
In 1998, two teams of astronomers independently discovered that the universe is expanding at an ever-faster rate, prompting physicists to speculate that something other than gravity was driving this process. Astronomers have since developed methods of studying this force: from studying explosions of white dwarf stars (commonly referred to as type Ia supernovae) and their explosions, to measuring ripples produced when black holes collide and merge and send out gravitational waves detected on Earth by observatories such as Laser Interferometer Gravitational Wave Observatory or its Italian equivalent Virgo.
Astronomers have turned to weak gravitational lensing as an additional technique, studying galaxies through images to detect any distortion caused by intervening objects' gravity and measure whether their shapes have been warped by it. Distortions can be quantified using statistically large samples of galaxy images; their maps have revealed evidence for dark matter presence.
No one really understands what constitutes dark matter; visible matter such as protons and neutrons only account for about five percent of the universe, leaving ninety five percent to be covered by non-baryonic dark matter? that does not interact with visible matter directly but may still interact indirectly via gravitation and weak nuclear forces such as those present. One theory suggests neutrinos may make up dark matter by flying through space at near light speed while only being affected by gravity or weak nuclear forces like gravitation.
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