milky way
An ancient galaxy might be hiding at the centre of the Milky Way. (Reuters)

The Milky Way could have an ancient galaxy hiding in its centre, with astronomers discovering millions of galaxies previously thought to be extinct.

These lost galaxies were found to have been concealed by discs of "stolen" stars, a study in The Astrophysical Journal has suggested.

A decade ago, astronomers found an excess of compact spherical galaxies in the early universe. They were about a third size of those found around our own galaxy and with a comparable mass, New Scientist reports.

These galaxies were common in the universe 11 billion years ago, but were believed to have gone extinct and replaced by huge elliptical galaxies (giant clouds of stars with almost no structure) and disc galaxies like our Milky Way and closest neighbour Andromeda.

Alister Graham, from Swinburne University of Technology in Melbourne, Australia, said almost all of the compact massive galaxies were thought to be missing from the nearby universe.

Scientists had thought these galaxies had been destroyed by mergers and collisions, but this led to a conundrum – that many mergers would result in a lot more of these galaxies orbiting one another ahead of a collision than we actually see.

After looking closer at surveys of galaxies in the local universe, scientists found 21 galaxies originally thought to be elliptical galaxies were actually flat disc galaxies with bulges in the middle – with exactly the same physical mass and size as those from the early universe.

Their findings suggest most of these 'missing' galaxies had, in fact, just grown a disk. Graham said: "The original, compact spheroid of stars remains basically unchanged in their centres. They were hiding in plain sight."

In addition, he said part of the Milky Way's central bulge could once have been one of these compact galaxies.

However, not all are convinced. Emanuele Daddi, from the French Alternative Energies and Atomic Energy Commission, who was one of the first scientists to notice the excess of spherical galaxies in the early universe, said: "The idea did not occur to us that they could actually be bulges of local [disc galaxies] that had not yet grown their discs. Neither did the few hundred papers that subsequently studied the problem consider this idea."