Dark energy and dark matter are so named because they are total mysteries to us. We don't know what dark energy or matter is, just that it's there. When it comes to the makeup of the universe, what we don't know hugely outweighs that which we do know.
It is thought that dark matter makes up 27% of the universe, dark energy 68%, and known matter, which makes up all stars and planets, constitutes only 5% of our universe. But how can we hypothesise how much of the universe is known when we don't know what makes up the rest of it?
In this article, we will explore what the terms 'dark energy' and 'dark matter' actually mean, as well as the theories attempting to explain the phenomena.
What is dark matter?
Dark matter is defined by NASA as "an invisible form of matter that makes up most of the universe’s mass". But what actually is it? Well, the reason why we call it 'dark' matter is because we don't know exactly what it is — we can't even see it.
Much like black holes, dark matter doesn't emit any light or energy, rendering it invisible and making it impossible to directly study it. Also like black holes, this means that we study dark matter by looking at the effect it has on matter around it. But what effect is this?
What does dark matter do?
We can see from its gravitational influence that dark matter has mass. It is thought that this gravitational influence "drives normal matter (gas and dust) to collect and build up into stars and galaxies". This is a pretty big idea: if this is true, it would mean that without dark matter, our galaxy — and every galaxy — may never have come to exist. We came to this idea from research findings, as studies of galaxies have shown gas and stars to move as though they are being influenced by much more mass than we can see; from this, we can infer that there is matter there which is invisible to us. As Yale University researcher Pieter van Dokkum puts it: "motions of the stars tell you how much matter there is ... they don't care what form the matter is, they just tell you that it's there".
Is there dark matter on Earth?
If there is invisible dark matter out there in the universe, what about here on Earth? In short, it depends on what you mean by 'Earth'. Given that dark matter makes up almost 30% of the universe, it is likely that there is dark matter in the Earth's crust.
Given the gravitational pull of dark matter, it seems that if there was a lot of it alongside us on Earth's surface then we would very likely be able to tell. So there is presumably dark matter on Earth — but it would probably be more accurate to say dark matter is in the Earth than on it.
Understanding dark matter
Considering that we study dark matter through its effects on nearby matter, it makes sense that we know what dark matter does: but what about the makeup of dark matter itself? Will we ever know it as any more than just 'dark'? Some scientists believe so.
There are a few emerging theories as to what dark matter could actually be — we've come a long way since dark matter first began to be understood in the 20th century.
Has dark matter been proven to exist?
Dark matter is accepted to have been first identified in 1933 by a Swiss astronomer named Fritz Zwicky. Using the virial theorem, an equation used to relate observable properties of a galaxy to the unobservable property of its mass, Zwicky 'discovered' dark matter.
Much more research has been done since then, but since no scientist has ever been able to directly observe dark matter, it has in that sense still not been proven. But given that dark matter merely refers to matter which we cannot see but know is there, it is likely that at least some of this matter exists.
There are some theories which suggest dark matter may not exist at all, arguing that instead, we lack a scientifically complete understanding of gravity — and that which we attribute to dark matter can actually be explained by the knowledge that we lack about gravity. While this idea is worth mentioning, it's also important to remember that the existence of dark matter is generally accepted in scientific circles, which agencies such as NASA and the European Space Agency (ESA) undertaking investigations into the nature of dark matter.
What is made up of dark matter?
There are a few theories as to what dark matter consists of. We seem to be getting closer to understanding dark matter all the time — but in the meantime, let's discuss some of the potential explanations.
Many of the theories attempting to explain dark matter involve hypothetical particles, such as WIMPS, sterile neutrinos, and axions. Weakly interacting massive particles, or WIMPS, are a "hypothesised class of particles to explain dark matter", theorised to make gamma rays after coming into contact with another WIMP. Sterile neutrinos are hypothesised to interact through gravity alone, which, of course, is the way we have been able to identify the existence of dark matter.
The axion particle is thought to have low mass and low energy, and is seemingly becoming the leading candidate in the race to understand dark matter. While none of these particles have been definitively discovered, it is probably very likely that at least one of them holds some answers regarding the nature of dark matter.
Primordial black holes
Primordial black holes are another hypothetical explanation. It is theorised that primordial black holes could have formed in the seconds following the Big Bang, due to the unique universal conditions which lasted for the seconds before the universe expanded. Primordial black holes could account for a large amount, if not all, of dark matter — black holes can't be seen directly and are only studied from their effect on nearby matter, much like dark matter. The discovery that primordial black holes exist would be a huge one, and it could entirely solve the mystery of dark matter.