Millions of ancient galaxies believed to have become extinct are in fact still around, astronomers have discovered, hiding in plain sight among discs of "stolen" stars.

The galaxies, known as compact spherical galaxies, were common throughout the early universe around 11 billion years ago, but looking around our local universe today suggests they disappeared from our cosmic neighborhood.

Computer simulations have suggested their seeming rarity today was because they'd been destroyed in collisions or mergers with other galaxies.

However, that assumption has one central problem: If collisions and mergers were that common, we should still see some of these type of galaxies orbiting one another and moving toward collisions today, but we rarely do.

"It was known that there are not enough mergers; this was an unexplained problem," says Alister Graham of Swinburne University of Technology in Melbourne, Australia.

In our cosmic neighborhood of the universe, it was assumed all compact massive galaxies had disappeared.

"Very few compact massive galaxies had been found locally, just a handful," Graham says.

Now, a new study suggests they haven't disappeared; they've just changed their appearance, which has allowed them to "hide" in plain sight, he says.

Re-examining cosmic surveys in our local universe, the researchers discovered many galaxies had been misidentified, or at least mischaracterized.

Graham and his colleagues found 21 galaxies originally thought to be large 3D gatherings of stars — in other words, your typical large elliptical galaxies — were in fact flat 2D galaxies with bulges in their centers. Those bulges? They are nothing less than the "missing" compact spherical galaxies, the researchers report in The Astrophysical Journal.

The bulges possess the exact same mass and compact size as the galaxies of the early universe thought to have disappeared, Graham explains.

The finding suggests the majority of compact spherical galaxies haven't disappeared; they've just grown a disk that could be hydrogen gas and stars "stolen" from smaller galaxies but without any merging, he says.

"The original, compact spheroid of stars remains basically unchanged in their centers," he says. "They were hiding in plain sight."

The number of such hidden systems approximately matches the number of compact massive galaxies that existed in the early universe, suggesting the same numbers of them are in fact with us today, say the researchers, who suggest there are 1,000 times more of these galaxies-within-galaxies in our local universe than previously believed.

"This explains why they had been missed; we simply needed to better dissect the galaxies rather than consider them as single objects," says study participant Giulia Savorgnan.

The phenomenon of changing their appearance might not be all that rare or unusual, Graham adds, suggesting at least a portion of the central bulge of our own Milky Way galaxy may once have been an example of these compact spherical galaxies.