Astronomers who study the expansion of the universe are interested in finding out just how fast the universe is expanding. For years, the big question was whether the universe was expanding fast enough to overcome the attractive pull of gravity. If yes, then the universe would expand forever, although the expansion would slow down over time. If no, then the universe would someday start to contract, and eventually would get squeezed together in a big crunch, the opposite of the Big Bang.

Recently, however, these astronomers have made a strange discovery: the rate at which the universe is expanding is actually increasing. In other words, the universe is expanding faster now than ever before, and in the future it will expand even faster! This answers the old question: the universe will keep expanding forever. But it also proposes a perplexing new question: what is causing the expansion of the universe to accelerate?

One possible hypothesis involves a new, as-yet-undiscovered form of energy called dark energy. We know even less about dark energy than we know about dark matter. However, some scientists believe that dark energy makes up more than half the total content of the universe. Other scientists have other hypotheses about why the universe is continuing to expand; the cause of the universe’s expansion is another unanswered question that scientists are researching.

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