Dark matter theory

The Universe we know it is made by Earth, the sun, other stars and galaxies.These are made by protons, electrons and neutrons. These are billions and trillions in number.But observations say that these matters make uo only 5% of the mass of total universe.The rest of the part is not blank. It is covered by invisible particle called Dark Matter (25%) and the force which repels gravity called  Dark Energy (70%).

Dark matter doesn't interact with light, so it is invisible. Other electromagnetic waves also don't interact with it. According to the scientists, these particles are found in the galaxies.Till now it is not observed yet. But it has gravitational effects.

If we think about our Solar system, the rotating speed of the
Mercury is much more than Neptune because Neptune is far away from the sun. It states that gravitational force becomes weaker when distances are increased (According to Newton and Einstein). So the speed of the stars which are situated at the edge of galaxies, should have less speed of spinning than core area.But it has been seen that the speeds are nearly equal. So it is assumed that the boundary stars are feeling the gravitational effects of an unseen mass—dark matter—in a halo around the galaxy. 

One theory says that it is an invisible particle that can't react with light but has gravitational force.

Another theory says that it is other form of gravitation which is only found in galaxies and it may be different than our known gravity.

Dark energy is opposite than gravity. Before 1990, scientists assumed that the gravitational force would be weaker over the time. So the expansion of the universe would slow down. But the experiment says that it is expanding in per second. The force seems to be growing stronger as the universe expands. This force is named as Dark energy.