Adam Riess, who is a researcher at the Space Telescope Science Institute in Baltimore and also a professor at Johns Hopkins University was one of the receivers of the 2011 Nobel Prize in physics for the finding that the universe is expanding at an accelerating rate using imageries of very aloof supernovae to derive what is now considered the most accurate measurement to date of cosmic speed. In a recent interview he said “Dark energy is incredibly strange, but actually it makes sense to me that it went unnoticed.
He continued “I have absolutely no clue what dark energy is. Dark energy appears strong enough to push the entire universe - yet its source is unknown, its location is unknown and its physics are highly speculative”
At the moment, there are numerous theories for the explanation of dark energy: it could be energy produced by ghostly subatomic particles that appear out of nothing before annihilating; it may related with the lately confirmed Higgs Field, which gives definite kinds of matter mass.
Or it can be clarified by string theory by which more invisible dimensions of space get crushed into sizes much smaller than atoms.
Some theorists even consider that it's an example of fine-tuning that requires the existence of a multiverse to explain it.
Researchers have established that our universe is expanding at a rate 20% faster than it was 5 billion years ago. In 1929, Edwin Hubble first proved that the universe was growing by presenting that galaxies outside the Milky Way, were moving actually away from each other.
Only about 5% of known the universe is consisted of planets, stars and gaseous structures, with the remaining 95% covering dark matter and dark energy. And dark Energy (whatever it is) is responsible for this expansion.
You can watch a video of Adam Riess explaining the universe: