Scientists say they've discovered huge, ancient galaxies that existed so soon after the universe's beginnings in the Big Bang, they call into question our understanding of how such large galaxies form.

Using infrared wavelengths to peer through the dust of the universe into its distant past, researchers say they've discovered 574 giant galaxies that were previously hidden from us despite their massive size.

The faint galaxies they've found date to when the universe was between just 0.75 and 2.1 billion years old, the scientists report in the Astrophysical Journal.

They're so old, they lack a defined shape, the researchers note; the elliptical and spiral galaxies we see all around us came later in the universe's timeline.

Their immense age and massive size are a problem for current theories of how large galaxies form, they say.

"We are talking about massive galaxies, twice as massive as the Milky Way today," says study lead author Karina Caputi of the University of Groningen in the Netherlands. "Currently, even the most up-to-date galaxy-formation models cannot predict such massive galaxies [before] almost 2 billion years after the Big Bang."

The numbers of these early but massive galaxies exploded in a very short period of time, already formed just three billion years after the Big Bang, researchers say.

"We found no evidence of these massive galaxies earlier than around one billion years after the Big Bang, so we're confident that this is when the first massive galaxies must have formed," says study co-author Henry Joy McCracken from the Institut d'Astrophysique de Paris.

However, the number that formed between 1.1 billion and 1.5 billion years following the Big Bang is many times what most theoretical models of galaxy formation predict, the researchers point out.

The leading current theory holds that galaxies form by accretion, merging lots of smaller groupings of stars in a hierarchical model, but that's a process that's just not fast enough to account for the numbers of massive galaxies existing so soon after the Big Bang, the scientists say.

"There's basically not enough time for these kinds of objects to form," McCracken said.

That so many have been found suggests large galaxies were much more common early in the Universe than was previously believed, and that many more are awaiting discovery, they say.

The new findings may mean that currently-held theories of how galaxies began forming in the early Universe may need a complete overhaul, they conclude.

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