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3D distribution of dark matter in the Universe

©NASA, ESA and R. Massey (Caltech)

Dark secrets of the Universe

It’s perhaps natural that we don’t know much about how the Universe was created – after all, we were never there ourselves. But it’s surprising to realise that when it comes to the Universe today, we don’t necessarily have a much better knowledge of what is out there. In fact, astronomers and physicists have found that all we see in the Universe – planets, stars, galaxies – accounts for only a tiny 4% of it! In a way, it is not so much the visible things that define the Universe, but rather the void around them.

Cosmological and astrophysical observations indicate that most of the Universe is made up of invisible substances that do not emit electromagnetic radiation – that is, we cannot detect them directly through telescopes or similar instruments. We detect them only through their gravitational effects, which makes them very difficult to study. These mysterious substances are known as ‘dark matter’ and ‘dark energy’. What they are and what role they played in the evolution of the Universe are a mystery, but within this darkness lie intriguing possibilities of hitherto undiscovered physics beyond the established Standard Model.

Dark matter

Dark matter makes up about 23% of the Universe. The first hint of its existence came in 1933, when astronomical observations and calculations of gravitational effects revealed that there must be more 'stuff' present in the Universe than telescopes could see.

Researchers now believe that the gravitational effect of dark matter makes galaxies spin faster than expected, and that its gravitational field deviates the light of objects behind it. Measurements of these effects show that dark matter exists, and they can be used to estimate the density of dark matter even though we cannot directly observe it.

But what is dark matter? One idea is that it could contain ‘supersymmetric particles’ - hypothesized particles that are partners to those already known in the Standard Model. Experiments at the Large Hadron Collider may be able to find them.

Dark energy

Dark energy makes up approximately 73% of the Universe and appears to be associated with the vacuum in space. It is homogenously distributed throughout the Universe, not only in space but also in time - in other words, its effect is not diluted as the Universe expands.

The even distribution means that dark energy does not have any local gravitational effects, but rather a global effect on the Universe as a whole. This leads to a repulsive force, which tends to accelerate the expansion of the Universe. The rate of expansion and its acceleration can be measured by observations based on the Hubble law. These measurements, together with other scientific data, have confirmed the existence of dark energy and provide an estimate of just how much of this mysterious substance exists.