If confirmed, this would represent arguably one of the biggest scientific breakthroughs for a hundred years, since Einstein's theories of relativity. That is because a fifth force and any particles associated with it are not part of the Standard Model of particle physics.

Researchers know that there is what they describe as "physics beyond the Standard Model" out there, because the current theory can't explain lots of things that astronomers observe in space.

These include the fact that galaxies are continuing to accelerate apart after the Big Bang that created the Universe, rather than the expansion slowing down. Scientists say the acceleration is being driven by an unknown force, called dark energy.

Galaxies are also spinning faster than they should, according to our understanding of how much material is in them. Researchers believe it's because of invisible particles called dark matter, which again are not part of the Standard Model.