Theorised 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.
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