Dark energy

Below is a figure I stole from Sean Carroll at the University of Chicago. For those who haven't yet heard, dark energy is a strange substance with negative pressure, whose existence has been recently inferred from cosmological observations. It seems to constitute about 70% of all the energy in the universe, and is causing accelerating expansion. (Don't worry though - nothing very noticeable will happen for the next few billion years.)

Theorists have no idea what this stuff is. Let me repeat that - theorists have no idea what this stuff is! The discovery, via supernova distance-redshift observations, was totally unexpected. That makes studying it tremendously exciting. However, we may not find out what it is for a long time. For example, dark energy might be a quantum field with very weak interactions, coupled to us only via gravity. In that case we might not learn much beyond how it affects the large scale evolution of the universe.

If the discovery of dark energy holds up, it will be one of the most surprising and important discoveries of the last century. The public hasn't really caught on to this yet, but who can blame them with all the background noise about speculative (meaning, very likely not real) things like superstrings, extra dimensions, supersymmetric partners, etc.