Dark matter is a form of matter that does not emit, absorb, or reflect light, making it completely invisible to telescopes. We know it exists because of its gravitational effects: galaxies spin fast enough that they should fly apart based on the visible matter alone, yet something with far more mass is holding them together.
Estimates suggest dark matter outweighs all the ordinary matter in the universe, stars, planets, gas, and dust, by roughly five to one. It does not seem to interact through electromagnetism at all, only through gravity and possibly the weak nuclear force, which is exactly why it has stayed so hard to detect directly.
Despite decades of searching, no experiment has yet confirmed what dark matter is actually made of. Leading candidates include entirely new, undiscovered particles, and ruling them in or out remains one of the most active pursuits in both particle physics and cosmology.