Matter can be converted to energy. Einstein came up with the equations, and we've proved it. Quantum mechanics shows how the line blurs at smaller degrees
In the beginning, there was an infinitesimally small ball of matter-energy existed, and something caused it to expand. It still is expanding. A lot of the matter-energy slid over to the matter side, and a lot slid over to the energy side.
There are 4 fundamental forces that act on everything within the universe: gravity, elecromagnetism, the strong force, and the weak force, the last 2 only affecting sub-subatomic particles. The latter 3 are all the same force in different forms, and we can mathematically connect them. To date, we have only been able to connect them to gravity via Higgs' theories. The Large Hadron Collider, being constructed underground in Europe will prove (or disprove) the existence of the Higgs Boson, a sub-subatomic particle that allows gravity to be another form of the electromagnetic-weak-strong force.
This is being done so physicists can show there is 1 type matter-energy particle that makes up all matter (and all the space between it), and 1 force that acts on it. (it acts differently under different conditions)