Really. When you suck on the end of a straw, a slight vacuum is created inside so you can get those last few drops from the bottom of a milk shake glass. But the vacuum of space contains a repulsive force that's so strong the universe has doubled in size as a result. Okay, that took 8 billion years, but who's counting?
The universe was thought to be unchanging, immutable, the "was," "is," and "will be," until Edwin Hubble showed in 1929 it was expanding. Since that time a major goal of astronomy has been to measure the rate of expansion and understand why.
During the late 1990s, observations by the Supernova Cosmology Project of a certain kind of exploding stars called "type one A supernovae" found that the universe was not only expanding, but--much to everyone's surprise--it was expanding at an accelerating rate. An example of a type Ia supernova is shown below on the outskirts of galaxy NGC 4526.
Recently, a team used the Hubble Space Telescope's Advanced Camera for Surveys to measure the properties of 23 distant supernovae. The results of their study just published by the Space Telescope Science Institute in Baltimore shows that the force is a fundamental and unchanging property of space itself. Einstein added such a force as a fudge factor to his general theory of relativity and called it the 'cosmological constant', but later decided it was just too weird to be true. But he was right to include it.
There's growing evidence that the source of this repulsive vacuum force is something called dark energy. This poorly understood energy represents almost 3/4 of the mass of the universe (remember, from E=mc2, energy and mass are the same thing). It's probably created by a seething mass of virtual particles that have brief existence in vacuum fluctuations. Indeed, one such quantum fluctuation may have been the spark that set off the Big Bang.