The first sighting of clustered dwarf galaxies bolsters a leading theory about how big galaxies such as our Milky Way are formed, and how dark matter binds them, researchers said Monday. Theorized but never seen, the bundled galaxies were discovered using the largest optical survey of the night sky ever compiled, they reported in the journal Nature Astronomy. Seven clusters of three-to-five galaxies are each 10 to 1,000 times smaller than the Milky Way. Unlike our home galaxy, all have long-since stopped giving birth to new stars. "We suspect these groups are gravitationally bound and thus will eventually merge to form one larger, intermediate-mass galaxy," said lead author Sabrina Stierwalt, an astrophysicist at the National Radio Astronomy Observatory in Charlotteville, Virginia. The findings shed light on several big questions about how structures such as galaxies formed in the early Universe, she told AFP. A leading theory predicts that, after the Big Bang some 13.7 billion years ago, smaller things joined together to form bigger ones.

But there has been frustratingly little observational evidence of such mergers occurring on a scale as small as dwarf galaxies, Stierwalt explained. One reason is that dwarf galaxies are hard to see. Only two -- known at the Magellanic Clouds -- are visible to the naked eye. As of a decade ago, no more than a dozen had been identified by astronomers. And even as bigger telescopes made their discovery commonplace, those found were either isolated "field dwarfs," or "satellite dwarfs" being cannibalised by larger galaxies. "Independent groups of only low-mass galaxies -- like the ones we found -- reveal a possible formation mechanism for larger ones such as our Milky Way," Stierwalt said. The clusters are between 200 million and 650 million light years away from Earth. "That sounds like a lot, but it is relatively nearby given the vast size of the Universe," she said.