1. Dark Energy

2. What is dark energy? Theorized force opposing gravity.
This force would accurately explain the current accelerating expansion of the universe.
Possible it might be related to the inflation period of the early universe.

3. Supporting Evidence
Measurement of supernovae type Ia, SN1997ff, actually predicted to be brighter than it appears in space, thus providing evidence that universe is expanding.

4. other Defining Properties
Dark Energy is said to be uniform through out space

5. Supporting Fact CMB anisotropy indicate the universe is flat.
Thus, matter density + energy density = critical density.
But, matter density only contributes 1/3 of the critical density!
So, in order for energy density to have escaped detection it must be distributed uniformly.

6. Importance of Dark Energy
Currently it is estimated that the composition of the universe is approx. 70% dark energy
25% dark matter
2-5 % matter (stars, H, He, neutrinos, heavy elements)

7. Importance of Dark Energy
Possibly predict the future of the universe.
If repulsion from dark energy becomes strong, could potentially lead to a Big Rip.
Or, a variable dark energy might fade away and then flip into a future that pulls the universe together rather then pushing it apart, into a Big Crunch

8. References Dark Energy and the New Cosmology,
Dark Energy by Wikipedia