Dark matter is an invisible substance that keeps galaxies from flying apart. If its particles occasionally decay near the Sun, they shoot out streams of fast particles, like invisible hammers striking light. Each impact turns a particle of sunlight into a gamma ray — a super-powerful flash that telescopes can spot.
Analysis of 15 years of Fermi telescope observations found no such gamma glow. This means dark matter particles live at least 10²⁷ seconds — billions of times longer than the age of the universe. It might seem like a lack of result is a failure, but in reality it rules out entire classes of theories. If they lived just 10 times less, the Sun would shine brighter in gamma rays than all the stars in the sky, and an amateur telescope with a gamma camera could detect it. This 'solar' method searches for dark matter right under our noses, without peering into distant galaxies.
If dark matter decayed just a bit faster, the gamma glow around the Sun could be seen even with an amateur telescope — though you'd need a special camera.