What is Dark MatterAuthor: Omkar
Introduction on what is dark matter:
Dark Matter and Dark Energy are hypothetical concepts hypothesized in the late 1990's to explain accelerating expansion of universe. Prior to the discoveries made possible by Hubble Space Telescope (HST) it was believed that expansion of universe has to stop at some point of time due to force of gravity. It followed that there should be at least some signs of slowing down.
HST made it possible to observe very distant supernovae with a startling discovery that the universe is actually accelerating its pace of expansion. There was no explanation for the observed phenomenon even though there had to be a cause.
Explanation of what is dark matter
Attempts to explain this discovery of accelerating expansion of universe led theorists to come up with conjectures. One possibility was that long-discarded version of Einstein's theory of gravity and its hypothesis of a "cosmological constant" may be correct. Second possibility was that there could be some strange kind of energy-fluid that filled space with the last possibility being that Einstein's theory of gravity may need to be revised to include some kind of field overcoming gravity and causing this observed cosmic acceleration.
The correct explanation is still unknown but the possible solution has been named as dark energy. Existence of dark energy in large quantity can be safely assumed due to its effect on expansion of the universe. But beyond that it is an absolute mystery. It is estimated to constitute about 70% of the Universe with its twin dark matter constituting about 25%. Everything else observable by instruments known to us is estimated to constitute less than 5% of the universe. May be dark energy and dark matter are the normal phenomena of space as what we consider normal and observable is a very small component of the whole.
Conclusion on what is dark matter:
It is easier told what dark matter is not than defining what it is. Theoretical models for the composition of the Universe designed to match actual observations have led scientists to estimate the ratios of dark energy, dark matter and normal matter (70:25:5) given above.
We are certain that dark matter is not in the form of observable stars and planets. It is also not in the form of dark interstellar clouds of matter and it is not antimatter either. It is also not in the form of large black holes because these can be estimated by the number of observed gravitational lenses and there are simply not enough such lenses to make up 25% of dark matter.
There do remain a few viable hypothetical possibilities. One of these is baryonic matter made up of the dark matter all tied up in brown dwarfs or possibly in small, dense chunks of heavy elements. These have been hypothesized as massive compact halo objects. But the generally accepted view point is that dark matter actually made up of some other kind of particles like Axions or Weakly Interacting Massive Particles (WIMPS).