With gasoline hitting $3 per gallon, scientists have just found the most energy-efficient engines in the universe — black holes, those whirling super-dense centers of galaxies that suck in nearly everything.

The jets of energy spurting out of older ultra-efficient black holes also seem to be playing a crucial role as zoning cops in large galaxies, preventing too many stars from sprouting.

That explains why there aren't as many burgeoning galaxies chock full of stars as previously expected, said scientists citing results from NASA's Chandra X-ray Observatory.

For the first time, scientists measured both the mass of hot gas that is being sucked into nine older black holes and the unseen super-speedy jets of high energy particles spit out, which essentially form a cosmic engine. Then they determined a rate of how efficient these older black hole engines are and were awe-struck.

These black holes are 25 times more efficient than anything man has built, with nuclear power being the most efficient of man-made efforts, said study lead author Steve Allen of Stanford University and the Stanford Linear Accelerator Center.

"If you could make a car engine that was as efficient as one of these black hole engines, you could get about a billion miles per gallon of gas," Allen said. "In anyone's book, that would be pretty green."

The galaxies in which these black holes live are bigger than ours, the Milky Way, and 50 million to 400 million light-years away. One light-year is nearly 5.9 trillion miles.

The black hole at the center of our galaxy wasn't studied because it wasn't gas-rich and big enough, so scientists couldn't measure what was going in and coming out, Allen said.

The results were surprising because the types of black holes studied were older, less powerful and generally considered "boring," scientists said. But they ended up being more efficient than originally thought — possibly as efficient as their younger, brighter and more potent black hole siblings called quasars.

Quasars spit out blinding light so scientists can't measure individual energy efficiency for them, said study co-author Christopher Reynolds of the University of Maryland. But if they could, they'd probably be even more efficient, based on indirect calculations, he said.

One of the ways scientists measured the efficiency of black holes was by looking at the jets of high energy spewed out. Those jets produce bubbles of heat nearby, which tend to keep hot gas from cooling and forming stars in large galaxies.

"The black holes are actually preventing galactic sprawl from taking over the neighborhood," said NASA astrophysicist Kim Weaver. She said there's no harm in too many stars, just a mystery of why these several billion old galaxies aren't loaded with even more stars.

Allen and Weaver said in interviews the unseen hot jets appears to answer the question about what's stopping galaxies from growing too big, he said.

"What this does is give us a step toward understanding why the galaxies in the universe look the way they do," Allen said.