Two different mysteries, often confused
Dark matter and dark energy are two separate, unrelated puzzles that happen to share a name because both were discovered through what they don’t do: neither emits, absorbs, or reflects light, which is how we detect almost everything else in astronomy.
Dark matter: the missing mass
In the 1970s, astronomer Vera Rubin studied how fast stars orbit at the edges of spiral galaxies and found something strange: they were moving far faster than the visible matter in those galaxies could gravitationally explain. Based on the visible stars and gas alone, outer stars should fly off into space entirely. Instead, galaxies hold together as if there’s substantially more mass present than what we can actually see.
The leading explanation is dark matter — some form of matter that interacts through gravity but doesn’t interact with light at all, making it completely invisible to telescopes. We only detect it indirectly, through its gravitational effects on galaxies, galaxy clusters, and even the bending of light itself (gravitational lensing). Despite decades of searching, no experiment has yet directly detected a dark matter particle — we know it’s there from its gravity, but not what it actually is.
Dark energy: the accelerating expansion
Dark energy is a completely different phenomenon, discovered in 1998 when astronomers measuring distant exploding stars (supernovae) found that the universe’s expansion isn’t just continuing — it’s accelerating. Something appears to be pushing space apart at an increasing rate, opposite to what gravity alone would predict (gravity should be slowing the expansion down, pulling everything back together).
Whatever is causing this is called dark energy, and it appears to make up roughly 68% of all the energy in the universe — dwarfing ordinary matter (about 5%) and dark matter (about 27%) combined. Unlike dark matter, dark energy doesn’t clump around galaxies; it appears to be a property of space itself, pushing outward uniformly everywhere.
The uncomfortable summary
Put together: everything humans have ever directly studied — every atom, every star, every planet — makes up only about 5% of the universe’s total content. The remaining 95% is dark matter and dark energy, and neither has a confirmed, complete physical explanation yet.
Why this matters
Massive modern research efforts — including large-scale galaxy surveys like the one covered in this site’s Research section — are specifically designed to pin down what dark energy actually is by precisely tracking how the universe’s expansion rate has changed over billions of years. Few open questions in physics are as large in scope, or as literally universal, as this one.