However, astronomers are still not sure what is causing the universe to expand. Because of this, astronomers call the mysterious energy causing the expansion dark energy. By studying the expansion of the Universe, astronomers have also realized most of the matter in the Universe may be in a form which cannot be observed by any scientific equipment we have. This matter has been named dark matter. Just to be clear, dark matter and energy have not been observed directly (that is why they are called 'dark'). However, many astronomers think they must exist, because many astronomical observations would be hard to explain if they didn't. 