what is dark energy? i heard that when i was watching a Tv programme about space.
Anything that gives vacuum a non-zero density can be called dark energy.
dark energy is supposed to be a counter to gravity, and it is supposed to cause the rapid expansion of the universe.
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.
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