Galaxies Without Dark Matter Might Form From Collisions

Galaxies without dark matter are rare and were once thought impossible. Find out how a new hypothesis about their formation may explain how all galaxies form and what dark matter is.

When I’m at the cottage in the summer, the night sky is usually clear and dark. That gives me a perfect view of the Milky Way on moonless nights. It’s a cloudlike, glowing belt of light that’s unmistakable between April and October here in the northern hemisphere.

Technically, every star in the sky is part of the Milky Way Galaxy. However, the luminous band stargazers call the Milky Way is a dense accumulation of stars between us and the galactic plane. It’s hazy because the stars are so close together that our unaided eye can’t resolve them individually.

The eclectic scientist Lord Kelvin was studying the Milky Way in 1884 when he came to a realization. He’d been trying to estimate its mass by measuring the velocity of the stars around the Milky Way’s galactic centre.

Answer Very Different from Mass of Individual Stars

He used that data to estimate the Milky Way’s garvity and from there its total mass. Inexplicably, the answer he got was very different from the figure he got by adding up the mass of all its individual stars.

That’s when scientists started musing about something began to call “dark matter” or “missing matter.” A range of prominent astronomers in the early twentieth century including Henri Poincare, Jacobus Kapteyn, Knut Lundmark, and Jan Oort all weighed in on the odd discrepancy.

The Swiss astrophysicist Fritz Zwicky was studying a group of galaxies called the Coma Cluster in 1933. He calculated that the cluster had 400 times more mass based on its gravitational effects than he could account for based on its visible stars. The missing matter clearly wasn’t any minor discrepancy.

We Can’t Account for 85% of the Universe’s Mass

We now know Zwicky’s estimate was on the high side. Even so, his basic finding was correct. We still can’t account for about…