what is dark energy?

[no idea]

what is dark energy? i heard that when i was watching a Tv programme about space.

[Fredrik]

Anything that gives vacuum a non-zero density can be called dark energy.

[Nenad]

dark energy is supposed to be a counter to gravity, and it is supposed to cause the rapid expansion of the universe.

[chroot]

Some clarification on Nenad's post:

Dark energy is a repulsive force that is responsible not for the universe's expansion, but for the acceleration of its expansion.

- Warren