Astrophysicists report how, when tiny galaxies collide with bigger ones, the bigger galaxies can strip the smaller galaxies of their dark matter — matter that we can’t see directly, but which astrophysicists think must exist because, without its gravitational effects, they couldn’t explain things like the motions of a galaxy’s stars.

It’s a mechanism that has the potential to explain how galaxies might be able to exist without dark matter — something once thought impossible.

It started in 2018 when astrophysicists Shany Danieli and Pieter van Dokkum of Princeton University and Yale University observed two galaxies that seemed to exist without most of their dark matter.

«We were expecting large fractions of dark matter,» said Danieli, who’s a co-author on the latest study. «It was quite surprising, and a lot of luck, honestly.»

The lucky find, which van Dokkum and Danieli reported on in a Nature paper in 2018 and in an Astrophysical Journal Letters paper in 2020, threw the galaxies-need-dark-matter paradigm into turmoil, potentially upending what astrophysicists had come to see as a standard model for how galaxies work.

«It’s been established for the last 40 years that galaxies have dark matter,» said Jorge Moreno, an astronomy professor at Pomona College, who’s the lead author of the new paper. «In particular, low-mass galaxies tend to have significantly higher dark matter fractions, making Danieli’s finding quite surprising. For many of us, this meant that our current understanding of how dark matter helps galaxies grow needed an urgent revision.»

The team ran computer models that simulated the evolution of a chunk of the universe — one about 60 million light years across — starting soon after the Big Bang and running all the way to the present.

Story Source: Materials provided by University of California — Irvine. Note: Content may be edited for style and length.