At the beginning of the 1990s, one thing was pretty certain about the expansion of the universe. It could have had enough energy density to stop its expansion and recover, it could have had so little energy density that it would never stop expanding, but gravity was sure to slow the expansion as time went on. Granted, the regression had not been detected, however, technically, the world had to slow down. The universe is full of matter, and the irresistible force of gravity binds all matter together. Then in mid-1998 and observations on a distant supernova made by the Hubble Space Telescope (HST) revealed that, a long time ago, the universe was actually expanding more gradually than it is today. So the expansion of the universe did not slow down because of gravity, as everyone assumed, it was accelerating. No one predicted that no one understood how to describe it. But there was something that triggered it.
Finally, researchers came up with three kinds of theories. Perhaps it was the result of a long-discarded version of Einstein’s theory of gravity, one that included what was called a “cosmological constant.” Perhaps there was some odd kind of energy-fluid that filled space. Perhaps there is something wrong with Einstein’s theory of gravity, and a new theory could include some kind of field that causes this gravitational acceleration. Theorists still don’t know what the correct answer is, but they gave a name to the solution. It’s called dark matter.
What is this Dark Energy?
More is hidden than is known to us. We know how much dark energy there is because we know how it impacts the expansion of the universe. Apart from that, it’s a complete mystery. But this is an important mystery. It points out that about 68% of the universe is dark energy. Dark matter is about 27 percent. The rest-everything on Earth, all that has ever been observed with all our instruments, all-natural matter-amounts to less than 5% of the universe. Come and think about it, maybe it shouldn’t be considered “natural” matter at all, because it’s such a small fraction of the universe.
One theory for dark energy is that it’s the product of space. Albert Einstein was the first person to understand that empty space is nothing. Space has many properties, many of which are only beginning to be understood. The first property that Einstein has discovered is that it is possible for more space to come into existence. Then a version of Einstein’s Gravity Theory, a version that includes a cosmological constant, makes a second prediction: “empty space” can have its own energy. Since this energy is the property of space itself, it would not be diminished as space expands. Unfortunately, no one in the scientific fraternity quite understands why the cosmological constant should be there in the first place, much less why it would have exactly the right value to cause the observed acceleration of the universe.
What’s the Dark Matter?
Considering the theoretical model of the structure of the expanding universe to the combined collection of cosmological observations, scientists have come up with the constitution mentioned above, ~68 percent dark energy, ~27 percent dark matter, ~5 percent normal matter. What’s the dark matter?
They are much more certain of what the dark matter is not than we are of what it is. First, it’s dark, which means that we don’t see in the form of stars and planets. Observations show that there is far too little visible matter in the universe to make up the 27% expected by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called barium. We say this because we would be able to detect baryonic clouds by absorbing the radiation that passes through them.
Second, dark matter is not antimatter, because we do not see the special gamma rays that are emitted when antimatter is annihilated with the matter. Eventually, we can rule out large black holes in the galaxy on the basis of how many gravitational lenses we see. As a result of high concentrations of matter tend to bend light while passing through it, but we do not see enough lensing events to indicate that such objects make up the required 25% contribution of dark matter.