Dark, but Light: Smallest Galaxies Ever Seen Help Solve a Big Problem

Source: W. M. Keck Observatory

DescriptionScientists have studied a population of the darkest, most lightweight galaxies known, each containing 99% dark matter. The findings suggest a major puzzle known as the “Missing Dwarf Galaxy” problem is not as severe as previously thought, and may have been solved completely.




Scientists may have solved a discrepancy between the number of extremely small, faint galaxies predicted to exist near the Milky Way and the number actually observed. In an attempt to resolve the “Missing Dwarf Galaxy” problem, two astronomers used the W. M. Keck Observatory in Hawaii to study a population of the darkest, most lightweight galaxies known, each containing 99% dark matter. The findings suggest the “Missing Dwarf Galaxy” problem is not as severe as previously thought, and may have been solved completely.

“It seems that very small, ultra-faint galaxies are far more plentiful than we thought,” said Dr. Marla Geha, co-author of the study and a Plaskett Research Fellow at the Herzberg Institute of Astrophysics in Canada. “If you asked me last year whether galaxies this small and this dark existed, I would have said no. I’m astonished that so many tiny, dark matter-dominated galaxies have now been discovered.”


The Missing Dwarf Galaxy puzzle comes from a prediction of the “Cold Dark Matter” model, which explains the growth and evolution of the universe. It predicts large galaxies like the Milky Way should be surrounded by a swarm of up to several hundred smaller galaxies known as “dwarf galaxies.” However, until recently, only 11 such companions were known to be orbiting the Milky Way. To explain this large discrepancy, theorists suggested that while hundreds of dwarf galaxies near the Milky Way may indeed exist, the majority might have few, if any, stars. If so, the galaxies would be comprised almost entirely of dark matter—a mysterious type of matter that has gravitational effects on ordinary atoms, but which does not produce any light. But proving the existence of a large number of nearly invisible galaxies seemed problematic, until now.