Why two terms "Dark Energy" and "Dark Matter"

[atuldpatil]

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?

 

[e.bar.goum]

Because the effects are different: Dark matter acts as if there is extra matter, resulting in galaxy rotation curves not expected from luminous matter, or gravitational lensing where you wouldn't expect. It acts just like other matter does, gravitationally.

Dark energy, on the other hand, acts to accelerate the expansion of the universe, which you can't get from just chucking extra particles around.

 

[Drakkith]

"Dark" just means that we have a hard time observing it, which is why both dark matter and dark energy are called dark. They are two distinctly different effects. Dark energy in particular shouldn't be thought of as normal energy, which has mass and causes attraction, but as a form of energy that has negative pressure, acting repulsively.