Dark matter

 Dark matter:                                                                                                                 In the early 1990s, everyone thought that expansion of universe is might slowing due to gravity. It is not Theoretically proved but the universe is full of matter and the attractive forces of gravity pulls all matter together. Later 1998,the Hubble Space Telescope came with observation of very distant supernova that showed that , a long time ago the universe was actually expanding slowly than it is today.                                                                                                                                                                                      Eventually theorists came up with three sorts of explanation  . Einstein's theory of gravity, one that contained what was called a cosmological constant may be there is some strange kind of energy fluid that filled space. May be there is something wrong with Einstein theory of gravity and a new theory could include some kind of field that creats the cosmic acceleration. Theorists still don't know what the correct explanation is but they have give the solution a name dark energy.                                                                                                                                               What is dark matter?                                                                                                                                                          The combined set of cosmological observations , scientists have come up with the composition that are 68%dark energy, 27% dark matter, 5% normal 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 made up of particles called baryons. We know this because we would be able to detect baryons  cloud 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 to make up the    required 25% dark matter contribution.                                                                                                                   However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or "MACHOs". But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).