Dark energy is responsible for the rapid acceleration of the universe, and encompasses the whole universe. It shouldn't be mistaken with dark matter, which behaves like glue and holds the universe together. Dark energy accounts for around 68% of the universe, whereas dark matter is roughly around 27%.
Any and every fact about the universe originates from the famous Big Bang theory. This theory is based upon the idea that the universe, from the time of its conception, is expanding rapidly. The universe was formed around 13.8 billion years ago after a violent explosion, which gives the theory its name. As the universe gradually cooled down, the billions of protons, neutrons, photons, and electrons, which were formed as a result of the big bang, combined to form hydrogen and helium. Further, hydrogen and helium combined to form giant clouds, that would form galaxies. During this period, some of the first stars were formed. As the universe grew older, planets and galaxies were formed, making the universe the way we see it today.
Imagine a triangular slice of pizza, and you can see the whole Big Bang theory in a glimpse; the way it started, from a point in the center, and is still expanding.
What is Dark Matter?
Have you ever wondered why the Earth doesn't just fly out of its orbit and wander around in the universe? Or, why don't galaxies split apart? The universe has its own provision to prevent collisions and disasters. It is true that the universe is expanding, but it is held together from the inside by invisible dark matter. Think about a Ferris wheel, the carriages are galaxy clusters, and the levers holding them is dark matter. Dark matter doesn't interact with electromagnetic forces, and is completely different from the normal visible matter.
There are still many mysteries about dark matter that need to be unfolded, but astronomers and physicists have gathered evidence about its presence. Recently, NASA's Hubble Space Telescope sighted more than a million globular clusters of stars moving together in the center of a giant group of galaxies, called the Abell 1689, providing clues for the existence of dark matter.
Dark Matter: Some Interesting Facts
Dark matter comprises around 84.5% of the total matter in the universe.
It isn't made up of baryons, unlike normal matter, which is a combination of protons and neutrons. Dark matter doesn't interact with electromagnetic forces. It doesn't absorb or emit light, nor does it reflect it like normal matter. The only way it can be detected is through its gravitational pull on visible matter.
Jan Oort, a Dutch astronomer, was the first to detect the presence of dark matter, in 1932. While studying stellar motions in nearby galaxies, Oort found that the matter seen in the galaxies is quite less as compared to the size of the galaxies.
The 'missing mass' problem suggested that, the stars, hot gases, and visible matter in the galaxy clusters accounted for hardly 20% of the total matter in the galaxy. The speed at which these galaxies moved was enough to flow them apart. According to Fritz Zwicky, a Swiss astronomer, these galaxies needed a lot more mass to hold themselves together, considering their speed. This meant that, there was some matter missing to the human eye, that was holding these galaxies together.
The strongest evidence for the presence of dark matter is provided by gravitational lensing studies of the bullet cluster. Collision of two galaxies was observed in this cluster, but it was astonishing that the galaxies merged together without any disaster, or the stars smashing into each other. This means that, there was some invisible gravitational field governing the stars of these galaxies.
A dark matter halo surrounds the whole galaxy and extends beyond its edges. The Milky Way galaxy is enveloped by a much bigger halo than the actual galaxy, and is spherical in shape.
Gravitational lensing is used to detect dark matter in the universe. Basically, the gravity of dark matter is responsible for bending light from distant light sources. To detect dark matter, astronomers check for bending of light, in cases of absence of visible matter.
Recent studies and observations seem to point toward a universe with a mixture of both hot and cold dark matter. The temperature determines the speed of the particles; hot particles move nearly at the speed of light. A good example is neutrinos. Neutrinos are weakly-interacting subatomic particles which do not carry an electric charge, hence, leaving them unaffected by electromagnetic forces. On the other hand, cold particles move much slower than hot particles. The hypothesis of the Peccei-Quinn theory in 1977 talks about 'Axions'. According to this theory, this hypothetical particle should have very low mass, and could be a possible component of cold dark matter.
Dark matter is just one of the several unknowns of space. A lot of research is being done on dark matter by the U.S. government. Projects like the Super Cryogenic Dark Matter Search (SuperCDMS), the LUX-ZEPLIN (LZ) Experiment, and the Axion Dark Matter Experiment Gen2 (ADMX-Gen2) have been initiated by the U.S. government for studying dark matter and its Weakly Interacting Massive Particles (WIMPS).