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universe. The remainder, ‘dark matter’, is not only dark, but also transparent and completely unaffected by radiation. It gives itself away only through its gravitational influence, altering the rate at which galaxies rotate and affecting the movement of galaxies within clusters.
The nature of dark matter is a mystery, but astronomers have ruled out some possibilities.
We can be pretty sure that it’s not just made up of small, dark objects that are undetectable through current instruments – for example stray planets, failed stars or black holes. Studies show that there simply aren’t enough of these to make much difference. Meanwhile, models of the matter’s distribution based on its gravitational effects show that it clumps loosely around visible objects. It’s not spread uniformly across the universe and it’s not an explanation for the relatively empty voids of space in the largescale cosmos. The current best guess is that dark matter is predominantly made from one or more unknown types of elementary particles, known as weakly interacting massive particles (WIMPs). This name says very little about what the particles actually are, and standard models of particle physics do little to address the question, but it seems that answers are most likely to emerge from ‘new physics’ discovered in experiments such as the Large Hadron Collider than through direct astronomical observations.
Meanwhile, there’s another even more elusive and troubling ‘substance’ in the universe.
The presence of so-called ‘dark energy’ was discovered in the late 1990s when astronomers found evidence that cosmic expansion is currently accelerating, rather than slowing down as we might expect due to the gravity of all the matter in the universe. Calculations show that it must account for around 71 per cent of all the energy in the universe. The nature of dark energy remains puzzling, and some still say the evidence for it is inconclusive. Among those who support the theory there are two popular explanations: it may be either an intrinsic property of space itself, or a ‘fifth force’ that acts like a form of antigravity on very large scales.
Fate of the universe
While 13.8 billion years is a long time, the universe is still young in some respects – some stars shining today formed alongside the first galaxies, and even our Sun is more than onethird the age of the universe. So what will happen to it in the future? On the largest scales, the fate of the cosmos is determined by the balance between its tendency to expand and the inward pull of gravity from all the matter it contains. Astronomers used to assume cosmic expansion would naturally slow down over time. This could lead to a universe that grew ever more slowly, with an effectively infinite life span during which stars would gradually process all of their available materials until they no longer had fuel to shine, and even matter itself might eventually disintegrate. In contrast to this ‘Big Chill’ scenario, gravity might win out and reverse the expansion of space, drawing everything back into a hot, dense ‘Big Crunch’. For a long time, estimates of the density of matter in the universe placed it on the fence between these two possible fates.
But the discovery of dark energy changes the game. Since it appears to accelerate expansion, it seems a ‘Big Chill’ is almost guaranteed. However, there is also evidence that dark energy has grown more powerful over time. If this is indeed the case, then expansion might continue to accelerate, overwhelming gravitation on ever smaller scales. This could lead to a ‘Big Rip’ in which galaxies, solar systems and eventually individual atoms are torn apart by the expansion of the space in which they exist. Fortunately, most measurements think such events are a long way off. Our universe, with all its wonders, will be around for a long time yet.