Early galaxies from the universe's distant past grew up much faster than astronomers anticipated, according to a new study in the current issue of Astrophysical Journal.

When the universe was two to three billion years old, or just 20 percent of its current age, these galaxies had already formed most of their stars.

"We expected to see that these galaxies had been in the process of star formation," study author Pieter van Dokkum of Yale University told SPACE.com.

The finding is somewhat perplexing to astronomers because they know that the universe is chock full of stars now (the Milky Way alone has 400 billion) and that very little star formation takes place in large galaxies today.

The stars "had to have formed at some time," said van Dokkum. But nobody knows when.

Based on the results of this study using the Gemini telescope in Chile, star birth in these large galaxies must have been "both much earlier and much more intense" than previously thought, van Dokkum said.

Scientists think black holes in these early galaxies' centers may have prevented stellar formation (and are suspected to be doing the same thing in galaxies today). As these dense masses devour all the matter around them, they shoot powerful jets of energy out into the surrounding space. These intense jets might heat the galaxy's gases, preventing them from condensing and forming stars - a sort of stellar birth control.

The only problem is that astronomers don't yet understand how the jets heat the gases. It's possible the jets aren't responsible at all, that some other energy source is at play, but astronomers don't have any other theories to work with.

"We don't understand exactly how this process works," van Dokkum said. "The problem is we don't have many other candidates."

Previous studies have implicated black holes both in promoting and preventing stellar formation. A study published last month in Nature found evidence that black holes could be blocking star formation in nearby massive galaxies, which also have few young stars.

But a study published in February of last year found evidence of a black hole jet triggering the collapse of a dense gas cloud, which became a stellar nursery.

"That's what makes it hard ─ we see evidence for both these things," van Dokkum said.

According to van Dokkum, the jets triggering star formation could be a short-term effect, like a sudden "shock", that later gives way to a net effect of heating, preventing the gases from condensing.

What astronomers do know is that something is stifling star formation because it is natural for galaxies to form stars since "there is a lot of gas available in the universe," according to van Dokkum. Small galaxies, which lack their large cousins' black holes, examined in a recent Hubble image were full of young stars, and many small galaxies today are still forming them.

Van Dokkum and his team hope to look back further in time, closer to the Big Bang, to look at the features of large galaxies then for evidence of star birth. They also plan to reexamine the galaxies in this study to try and figure out whether the black holes are in fact acting as "cosmic contraceptives."