Dark Matter Puzzles Stargazers Until Now

South Pole Telescope— The light from the Big Bang fills the sky with a cosmic hiss in the microwave part of the electromagnetic spectrum. The South Pole Telescope (SPT) is tuned to this afterglow. The map of the hot and cold spots on the sky made by astronomers using the SPT lets them find the mass of the dark matter in the universe to within a percent. Here, cosmic rays from space slam into the air above the SPT (glowing blue-green) to make a pink glow.

Just after the 2025 March Equinox, astronomers will gather in Los Angeles, California for the 15th Dark Matter Symposium from March 24-27, 2025 at the University of California, Los Angeles (UCLA). Held every couple years since the 1990s around the March Equinox in Los Angeles, the Dark Matter Symposium focuses on the stuff between the stars that holds the Milky Way together. In the early 1960s, the astronomer Dr. Vera C. Rubin and her astronomy graduate students at Georgetown University in Washington, D.C. tracked the way 500 of the hottest stars in the sky were moving. Rubin and company found that the hot stars were moving too fast for the galaxy to hold together if the only source of gravity in the galaxy are its stars.

Beginning in the early 1990s, telescopes began to map out the hot and cold spots on the sky from the light left over from the Big Bang. The size of the spots are not all the same and some spot sizes are much more common than others. From this “spectrum” of the number of cold and hot spots of different sizes on the sky, astronomers worked out the shape of the universe and found it to be “flat” in the sense that the angles of a triangle add up to 180 degrees to within a degree or so. When combined with measurements in the nearby universe of the way that the light from galaxies gets redder the farther away they are, the astronomers were able to back out the amount of matter that fills the universe as a whole.

By the mid-2000s, a revolution had taken place in astronomy. Astronomer’s were now convinced that it was not just our own Milky Way that is filled with stuff other than stars. The whole universe seems to be filled with this dark matter as well. In fact, as early as the 1930s, in the hills above Los Angeles, astronomers using the largest telescope in the world at that time, came to realize already back then — almost a hundred years ago — that the space between galaxies is filled with dark matter.

So what is dark matter? That is the thing every one wants to know. For thirty years, astronomers have met in Los Angeles at UCLA every other year or so around the March Equinox to figure out the nature of dark matter. This year, at last, the answer will be revealed.

Science Synergy Science Chair Dr. Noah Bray-Ali will give a 90 second “poster” talk on Tuesday night March 25, 2025 at the Symposium. The short talk will advertise the measurement of the mass of the particles that form the dark matter in the universe to 44 parts per million precision (See Infrared Light Makes Dark Matter in Plastic with Soundfor the details of the measurement). When combined with the fact that there are six of these particles — known as axions — for every particle of light made in the Big Bang (See Making Dark Matter in the Big Bang for the axion-to-photon ratio), the results of the measurements imply that the dark matter in the universe is made of these particles.