Observations of very distant exploding stars show that the expansion of the Universe is now speeding up, rather than slowing down due to gravity as expected. Over the largest distances, our Universe seems to be dominated by a repulsive "dark energy" of unknown origin that stretches the very fabric of space itself faster and faster with time.
Alex Filippenko (NAS), University of California, Berkeley, was a member of both teams that discovered in 1998 the accelerating expansion of the Universe, driven by "dark energy."
