Dark matter — the glue of galaxies
In the 1970s, astronomer Vera Rubin measured the rotation speeds of stars in galaxies. By Newtonian gravity, stars far from the centre — where most visible mass is concentrated — should orbit slower, just as Pluto orbits the Sun far more slowly than Mercury. Instead, stellar rotation curves are flat: stars at the edges orbit just as fast as those near the centre.
The only explanation consistent with all observations is that galaxies are embedded in enormous, invisible halos of matter — roughly 5–10 times more mass than the visible stars and gas. This dark matter exerts gravitational force but emits, absorbs, and reflects no light. It is not black holes or dim stars; Big Bang nucleosynthesis limits baryonic matter to ~5% of the universe. Dark matter must be something entirely new — possibly a weakly interacting massive particle (WIMP) or an axion. See the full evidence in our what is dark matter guide.
Dark energy — the accelerating universe
In 1998, two teams studying Type Ia supernovae expected to find the universe's expansion slowing under gravity. Instead they found distant supernovae dimmer than expected — farther away than a decelerating expansion would place them. The expansion is accelerating. Some unknown energy fills space and acts against gravity on the largest scales. Named dark energy, it makes up ~68% of the universe's total energy content.
The simplest model is Einstein's cosmological constant Λ — a fixed energy density of empty space. Whether it is truly constant is the central question being tested by the ESA Euclid mission and the DESI galaxy survey. Our news article on the Euclid dark universe map covers the first observational results.