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Scientists searched for dark matter signals in the light from dwarf galaxies, and found nothing.
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Dark matter is like an invisible glue that holds galaxies together. It emits no light, but if its particles collide, gamma rays can flash. Scientists hunted for such flashes in dwarf galaxies — small stellar clusters with lots of dark matter and almost no ordinary matter. It's like listening for a whisper in a quiet room: fewer distractions make it easier to hear.
The HAWC observatory in Mexico is a huge water tank that catches particles from gamma rays. The team improved their data analysis and expanded their list of dwarf galaxies. But even after all those upgrades, no signal appeared. That means dark matter particles collide even more rarely than we hoped.
HAWC isn't a telescope: it's a water tank at an altitude of 4100 meters. Gamma rays create faint flashes of light in the water, which sensors detect.
The result is new limits on how often dark matter particles collide. For masses from 1 to 10 000 TeV (thousands of times heavier than a proton), the upper limit is $10^{-23}$ cm³/s. That's like two dark matter particles meeting once in a trillion years in a cubic centimeter of space.
Dark matter remains invisible, but the search goes on. Every 'no' narrows the range of possible theories. Maybe next time we'll get luckier.
Dwarf spheroidal galaxies are almost entirely dark matter — ordinary matter makes up less than one percent of them. They are the darkest known places in the universe.
