The Universe is composed essentially of energy and matter. Matter makes up about 28%, while the remaining 72% or so is made up of dark energy. Dark energy is as yet a poorly understood property of the Universe, but its role is to explain the accelerated expansion of the Universe.
Of the 28% of matter in the Universe about 18% is what we observe as ordinary matter – which is what we can explain using particles such as electrons, protons and neutrons. That 18% really translates to just about 5% of the whole composition of the Universe! All that we call ‘ordinary matter’ – all the objects we can touch and see and feel, all the animals and plants, all of the planets, stars and galaxies, all of the known, observable stuff in the Universe only accounts for a mere 5%! The remaining 23% of matter which we cannot yet detect but whose existence must be accounted for is what we call dark matter.
It is called ‘dark’ because it does not interact with light or radiation as ordinary matter does. As such, to observe dark matter is extremely difficult. We can only infer the presence of dark matter not from direct detection but from how we see galaxies move and how they emit certain high energy radiation. These peculiar behaviours of galaxies can, so far, only be explained if they were larger than what we can actually observe. To account for that extra, invisible mass, we have postulated the existence of dark matter. We know it’s out there but we just can’t see it!
In summary, the Universe is composed of nearly 5% ordinary matter, 23% dark matter and about 72% dark energy. There are several projects and experiments in place to detect this elusive dark matter, and detecting it would solve one of the greatest mysteries of our Universe.