Dark matter and dark energy have proven to be remarkably elusive, resisting all efforts so far to be directly detected. The best evidence we have for their existence is indirect, through gravitational effects that we have ascribed to their presence. The problem is that the gravitational force is both very weak (the weakest of the four fundamental forces) while at the same time, in the presence of huge masses like the Earth, stars, or galaxies, its effects are also large, dwarfing the effects of other forces. But such indirect evidence for the existence of fundamental particles is never satisfying because scientific history has examples where that has led us astray. So the search goes on, with the construction of evermore sensitive detectors that we hope will finally provide convincing direct evidence.

One of the latest efforts is to send detectors into space.

In just a few weeks, a remarkable European probe will be blasted into space in a bid to explore the dark side of the cosmos.

The €1bn (£850m) Euclid mission will investigate the universe’s two most baffling components: dark energy and dark matter. The former is the name given to a mysterious force that was shown – in 1998 – to be accelerating the expansion of the universe, while the latter is a form of matter thought to pervade the cosmos, provide the universe with 80% of its mass, and act as a cosmic glue that holds galaxies together.

Both dark energy and dark matter are invisible and astronomers have only been able to infer their existence by measuring their influence on the behaviour of stars and galaxies.
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Scheduled for launch on 1 July, Euclid will take a month to cross the solar system to its destination 150 million km from Earth at a position known as the second Lagrange point. Here the craft will be able to peer out into deep space with the sun, Earth and moon behind it. The 2-tonne spacecraft will then begin its survey of the heavens.
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Such precision will be crucial in uncovering the secrets of dark matter, which cannot be seen directly because it is most probably made up of particles that do not emit, reflect or absorb light, according to scientists. To get around this problem, Euclid will exploit a phenomenon known as gravitational lensing.

This will involve taking millions of images of galaxies. In some cases, light from these distant bodies will pass through dark matter on its journey towards Earth. When that happens, its gravitational field will stretch and deflect the path of the light. This is gravitational lensing and the distorted images it produces will provide key insights into the nature of the dark matter that is triggering them.

But this evidence will still be indirect, by measuring the gravitational effects. It will be less than the gold standard of particle physics which involves postulating particles that have specific properties that can tested by their interaction with other particles.

When particles prove to be so elusive, there comes a nagging fear that they may not exist at all and that we are looking for a mirage. How and when one decides to give up the search is not easy to specify a priori. After all, it is impossible to prove that a particle does not exist. In the case of the aether which was similarly strongly believed to exist but was also elusive, the current belief in its nonexistence was a consequence of the theory of relativity making its existence redundant. In the case of dark matter and dark energy, we do not as yet have a consensus on alternative theories that might make them redundant, though Modified Newtonian Dynamics (MOND) has been gaining ground in recent years as an explanation that makes dark matter redundant.

Whenever searches for something keep turning up negative, alternative theories emerge that seek to explain away those results. This was the case with the Higgs field and the Higgs particle that were similarly searched for for decades until finally detected at CERN. As another example from earlier decades, after the the neutrino was postulated, it was also highly elusive and during the time before it was directly detected, alternative theories, that suggested that the conservation of energy and angular momentum need not always hold, were proposed. That state of uncertainty continued until the neutrino was directly detected.

So that is where we stand right now with dark matter and dark energy. I myself am agnostic on the question of whether they exist. Postulating their existence does explain many features of cosmological data but the absence of direct detection makes me a little uneasy as well.