Scientists at NASA say they've been investigating an odd, cyclic pattern of light coming from two distant black holes locked together in a complex gravitational dance that will likely see them eventually merge.

Researchers at the agency's Jet Propulsion Laboratory in California say the "funky" light — their word — is some of the strongest evidence to date for the existence of such merging pairs of black holes.

Ground-based telescopes first detected the pair of black holes, together dubbed PG 1302-102, earlier in the year.

They've been confirmed as the tightest orbiting duo of black holes ever detected, separated by a distance not much more than the diameter of our own solar system.

That closeness suggests they'll collide and then merge in around a million years — a brief interval in the cosmic scale of time — with a resulting huge blast throwing out the energy of 100 million supernovae, JPL researchers say.

The merging of giant black holes at the center of galaxies was a common event in the early ages of the universe, they say, but despite being common, they've proven difficult to detect and confirm.

Astronomers were led to PG 1302-102 by an unexpected light signal emanating from the center of a distant galaxy, found in data from NASA's Galaxy Evolution Explorer (Galex) and confirmed by observations using the agency's Hubble Space Telescope.

"We were lucky to have Galex data to look through," says David Schiminovich from Columbia University in New York, co-author of a study on the black hole pair appearing in Nature. "We went back into the Galex archives and found that the object just happened to have been observed six times."

The oddly cyclical signal is probably being created as the two black holes swing around each other every five years, they say.

Black holes don't emit light, of course, but they affect material around them that does give off light, the astronomers point out.

One of the black holes in the orbiting pair is apparently sucking up more matter than the other one, heating up surrounding matter that is emitting energetic light.

That light cycles from brighter to dimmer as the black hole moves toward us and then away from us, study lead author Daniel D'Orazio of Columbia University explains.

"It's as if a 60-watt light bulb suddenly appears to be 100 watts," he says. "As the black hole light speeds away from us, it appears as a dimmer 20-watt bulb."

The findings should help in understanding other merging black holes across the universe, which are only now beginning to yield their secrets, the researchers say.