Could a new, fifth force of nature provide some answers to our biggest questions about dark matter and dark energy? We’re working on it.

The Standard Model is, for all intents and purposes, the supreme accomplishment of modern physics. It describes four forces of nature, a zoo of particles, and how they all interact. It is perhaps the most successful scientific theory of all time.

And it’s fantastically incomplete.

It turns out that the Standard Model is able to account for less than 5% of all the matter and energy in the cosmos. Another 25% or so is Dark Matter, an unknown kind of matter that is for all intents and purposes invisible. The rest is known as Dark Energy, a mysterious entity that is causing the expansion of the universe to accelerate.

One of the first things astronomers noticed when they first discovered dark matter and dark energy was their apparent similarity. Why in the world are the two dark components of our universe roughly the same strength? I know, 25% and 70% don’t sound very similar, but when it comes to astronomy – and especially cosmology – they’re basically the exact same number.

Maybe it’s just a coincidence that they have about the same strength, and we’re overthinking it.

Or maybe it’s something else. Clever physicists have proposed connections within the “dark sector” of the universe, where dark matter and dark energy talk to each other. This would allow them to follow each other’s evolution, ensuring that they have roughly equal contributions to the energy budget of the universe for long periods of time.

To make them talk to each other, you need a force. But this force can’t be any of the known ones, otherwise dark matter and/or dark energy must also interact with normal matter, and we would have seen more directly evidence of them already.

So it has to be a new force, a fifth force of nature, completely different from electromagnetism, gravity, strong nuclear, and weak nuclear.  While ideas like this remain only in the realm of hypothesis, some of the ideas already have names.

One name is quintessence, the fifth essence of the universe. Another is dark photons, a particle that travels the cosmos like a photon but is, as its name suggests, dark.

To test these ideas we have to turn to the cosmos for answers. If a fifth force exists, it must be very subtle. Stronger manifestations of the fifth force have already been ruled out by observations of galaxy clusters, the expansion of the universe, and even the behaviors of neutron stars. So we have our work cut out for us – it will take a truly massive amount of data to tease out some signal that differs from expectations.