What can be observed only by observing its affects on gravity?

Dark matter

Explanation:

Very little is known about dark matter. This is because this matter does not interact with electromagnet waves, meaning it does not emit, reflect or absorb any kind of electromagnetic radiation that we know and is therefore not visible even by telescopes – hence it being named ‘dark’. However,  this matter has been observed due to its gravitational effect it is having on visible matter. This matter is believed to be responsible for pulling on galaxies even when evidence shows that the universe is expanding and therefore galaxies should be moving away from each other. Incorporating dark matter in astronomical calculations helps make sense of the seen motions of the galaxies and other observable matter in the universe because otherwise, things would not add up.