As I understand "Dark Matter" is an _assumption_ of existence of matter in certain space, to explain gravitational effects on the matter around that space. But when it comes to "Dark Energy" it is another assumption to explain why universe is expanding. But by Einstein's theory, mass and energy is same thing. Then why can't energy in the form of "Dark Matter" itself be assumed to explain expansion of universe? Won't it make simpler than assuming matter and energy separate?