Astronomers using the W. M. Keck Observatory on Maunakea, Hawaiʻi Island, have identified a third galaxy that appears to contain no dark matter. This discovery adds to a small but puzzling class of objects that challenge standard models of galaxy formation.
The galaxy is part of a peculiar linear structure that researchers believe may have formed during a violent collision between galaxies. The finding was made possible by precise measurements of stellar velocities, which revealed the galaxy's mass is entirely accounted for by its visible stars and gas.
Dark matter, an invisible substance thought to make up about 85% of the universe's matter, is typically invoked to explain why galaxies rotate faster than their visible mass can account for. The existence of dark matter-deficient galaxies suggests that dark matter may be separable from normal matter under extreme conditions.
The discovery was led by a team using spectroscopic observations from the Keck Observatory's Low Resolution Imaging Spectrometer. The origin of the galaxy's linear structure remains unclear, but simulations suggest it could be a tidal feature from a past merger.
This finding deepens the mystery of how galaxies without dark matter form. The previous two such galaxies were also discovered within the last few years, hinting that they may be more common than initially thought.