The early 1990s had people certain about one thing regarding the expansion of the Universe: Once it gained enough energy density, the expansion would stop and the universe would recollapse / The energy density would drop to a minimum value, such that gravity would superrule it and cause contraction of the system due to attraction. Great! “What’s the issue?” one would ask. Well, this was just a theoretical explanation. But the things turned out to be weird after the Hubble was sent up in 1998 and its observations recorded.
Observations of very distant supernovae showed that the universe was actually expanding much slowly, a long time ago, than now. So, the expansion of the universe had not been slowing down due to gravity as everyone had predicted, but instead accelerated over time. No explanation could be given for this, but we knew that something was causing it.
Soon, three theories were developed to explain the observations:
- It might have been a result of the long-discarded version of Einstein’s theory of gravity- the one which contained what was called the “cosmological constant”
- There might have been a strange kind of energy fluid that filled space.
- Or, the present ‘Einstein’s Theory of Gravitation’ was wrong and needed to include a field that caused cosmic acceleration.
More is unknown about dark matter and dark energy than is known. We can only calculate how much dark energy is present in the universe on the basis of how it affects universe’s expansion. So, it turns out there is roughly 68% Dark Energy, 27% Dark Matter, and the rest, less than 5% of the observable matter that we see on earth.
Albert Einstein was the first person ever to realize that ’empty space’ did not mean ‘nothing’. The first property he discovered was that, more space can come into existence. The cosmological constant containing theory by Einstein makes another prediction, viz. “empty space” can possess its own energy. Moreover, it would never fade or get diluted, as it was a property of space itself. So, as space came into existence, so did the new energy associated with it. But, no one could understand the reason for a cosmological constant to even be present, and if at all, be in the proportion it is, to cause acceleration of expansion of the universe.
Another theory states that the space might have been filled with particles that continually form and disappear. Unfortunately, when calculated by physicists, the energy it would give empty space, the answer came out to be 10120 times too big! That is a lot of error, indeed!
The second theory states that empty space has dynamical energy fluid. Theorists refer to it as ‘quintessence’. But if that’s what we are looking for, we anyway do not know what quintessence is like, what it reacts wit, et cetera.
Lastly, some theorists say Einstein’s Gravitation Theory is wrong. But then, what theory will it be, that describes all the things correctly explained by Einstein, like movement of planets in solar system, clustering of galaxies, etc. as well as the Dark energy phenomena. There are candidate theories, none compelling enough, whatsoever.
To jump to a conclusion, we need more, better data.
Dark matter, on the other hand, is a much more “unknown” topic than Dark energy. We are very certain of what dark matter is NOT, rather than knowing what it is! Here’s what we have on Dark Matter:
- Firstly, it is dark, hence NOT in the form of stars and planets.
- Second, it is NOT in form of dark clouds of normal matter/matter made of particles called baryons. We can be certain of this, as dark matter doesn’t show absorption of radiation.
- Third, dark matter does not release gamma rays that is done by antimatter on annihilation with matter. Hence, it is NOT Antimatter.
- Next, it can NOT be galaxy sized black holes, on the basis of gravitational lenses we see.
All these points considered, we still have candidates for what can be Dark Matter. For example, Baryonic matter can behave as dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. However, the most common view is that dark matter is not baryonic matter at all, but something much more exotic like axions and WIMPS (Weakly Interacting Massive Particles).