The spiral galaxy M99, imaged by the Hubble Space Telescope
ESA/Hubble & NASA, M. Kasliwal
Many large galaxies were long thought to be missing a huge proportion of matter – but it has now been found, and that is an even bigger problem for our understanding of the universe.
Observations over the course of the last decade or so have shown that galaxies about the size of the Milky Way appear to have far less baryonic matter – meaning normal matter, not dark matter – than we expect. This expectation is based on the ratio of dark …