Dark matter and dark energy

About 25% of the Universe appears to be made up of invisible dark matter. The presence of dark matter can only be ‘seen’ by how it affects the apparent shape of galaxies. Light from a distant galaxy is bent and distorted due to the presence of clumps of dark matter, making the galaxy look slightly warped. This effect is called weak gravitational lensing.

One of Roman’s main tasks will be to scan about 12% of the sky high above our galactic plane to look for the lensing effect in millions of distant galaxies. By studying tiny changes in the shapes of galaxies, Roman will help astronomers accurately map the distribution of ordinary and dark matter across the history of the Universe and trace how galaxies evolved.

Roman will investigate how the Universe has expanded over time. The expansion appears to be speeding up, and dark energy seems to be the culprit. Scientists think that dark energy makes up about 70% of the Universe and yet its nature remains a mystery. Like ESA's Euclid, Roman will use multiple methods to help us understand what dark energy is and how it has evolved over time.

In one method, Roman will look for exploding stars known as supernovas, in distant galaxies, especially type Ia supernovas. Because these explosions peak at a predictable brightness, astronomers can use them to measure cosmic distances, which in turn provide insights into how fast the Universe is expanding.

In another method, Roman will map how galaxies cluster together in time and space to test the effect of dark energy on them. Roman will study baryonic acoustic oscillations – imprints of sound waves that once rippled through the very early Universe.