Recent data on fluctuations in the intensity of the CMB (an all-sky wash of microwave energy that is the oldest observable result of the Big Bang) fits theoretical models of a universe which will continue to expand forever. Scientists had puzzled about what would keep the expansion going; dark energy seems to provide the answer.

But that doesn’t mean dark energy has made life easy for cosmologists. For one thing, they still have to figure out what it is. "It’s important to realize that the dark energy is different from any other kind of energy we’ve ever found," says Hogan.

"Presumably, if we ever get a truly unified theory of everything, which includes gravity and the other forces of nature, one of the big tests of that is, does it predict dark energy? Does it get that right or not?"

In the meantime, astronomers look forward to observations by satellites designed to map the Cosmic Microwave Background at higher sensitivity and finer resolution. Those data should reveal more about the expansion of the universe and the role of dark energy.

"They’re now down to below a millimeter," he says. "If they get down to a tenth of a millimeter, they’re actually at a scale where they’re testing some models of the dark energy."

And Ellis stresses the importance of further observations to confirm the supernova results and their stunning implications.

"If the supernova results were not to hold up … because supernova were found to be different at early times than today -- perhaps they're dimmer for some reason we don't understand -- then the universe wouldn't be accelerating," Ellis says. "I think it's unlikely, but I think it's so important that we have to check. It's such a big claim, and it's so counter-intuitive that the universe would be accelerating, in my opinion, that no stone should be left unturned. We should verify this as best as we possibly can."