Going by our basic knowledge about the nature of the universe, we believe that the universe had enough energy density that it became fairly clear that the universe was expanding. Now, it is known that the universe is full of matter. Owning to the gravity of matter present in the universe, this expansion was guaranteed to slow down as time went on. No conclusive proof of this fact was ever found but theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together.
Then in 1998, something strange happened. The Hubble Space Telescope (HST), while taking pictures of a distant supernova, compared the brightness of the object and its distance after taking the redshift into account, conclusively showed the universe was expanding at an accelerating rate. There was nothing that the scientists could use to explain it. Something was causing the expansion of the universe to accelerate and no one knew what it was.
Eventually, some scientists were able to explain this acceleration using ‘Dark energy’.
Now, what is dark energy? Dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe. It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the Universe.
The first explanation about the role of dark energy in the expansion of the universe comes by attributing dark energy as a property of space. As the universe is expanding, more space is coming into existence. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster.
Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. They do not necessarily exist but their presence is a way of explaining the expansion of the universe. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out too big and the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence”. It is a scalar field. Quintessence differs from the cosmological constant explanation of dark energy in that it is dynamic, that is, it changes over time, unlike the cosmological constant which always stays constant. It is suggested that quintessence can be either attractive or repulsive depending on the ratio of its kinetic and potential energy. Specifically, it is thought that quintessence became repulsive about ten billion years ago.
When all these possibilities yield no confirm answer, one turns to the fact that maybe Einstein’s theory of gravity is not correct.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.
As mentioned earlier, 27% of the universe is thought to be made of dark matter. What is dark matter? First, it is dark, meaning that it is not in the form of stars and planets that we see. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects make up the required 27% dark matter contribution.
Dark matter is another explanation that is thought to explain the accelerated expansion of the universe. As the universe expands, the radius of the observable universe decreases and going further, all the particles will be separated as there will come in existence a force greater than the force of bonding and the space between particles will increase further increasing the amount of dark matter in the universe that will further accelerate the expansion of the universe.