Dark Matter Explained: The Invisible Mass Shaping the Universe
Dark matter is the name astronomers give to the invisible mass whose gravitational influence is felt throughout the universe. It does not emit, absorb, or scatter light, so telescopes can't photograph it directly. Yet multiple independent lines of evidence point to its existence.
The Evidence
In the 1930s, Fritz Zwicky noticed that galaxies in clusters were moving far too fast for the visible matter to hold them together. Decades later, Vera Rubin and others measured the rotation curves of individual galaxies: stars at the edges orbit as fast as stars near the center, which only makes sense if each galaxy sits inside a large halo of unseen mass.
Other evidence comes from gravitational lensing — light bent by mass that isn't visible — and from the cosmic microwave background, whose fine details are best explained if most of the universe's matter is dark.
What It Might Be
The leading candidates are weakly interacting massive particles (WIMPs) and axions, both hypothetical. WIMPs would interact via gravity and maybe the weak nuclear force but not electromagnetism. Axions are extremely light particles proposed to solve a separate puzzle in particle physics.
Crucially, physicists know how much dark matter there is — about five times more than ordinary matter — without yet knowing what it is. Experiments deep underground and at particle accelerators continue to search.
This article is an accessible introduction. Cosmology is an active field, and specific candidates may evolve as new data arrives.