The largest contribution to the mass of the universe is believed to come from dark matter (i.e. matter that we can’t see). This dark matter is inferred to exist from studies of the motion of outlying stars, orbiting galaxies, the motion of galaxy clusters and from images formed by gravitational lenses (composed of clusters of galaxies) of background galaxies. All these cases show a massive ring of invisible matter extending well beyond the visible material in galaxies and contributing a factor of around 5 to 10 times the visible mass density to the mass of the universe.
The dark matter is spread a bit more thinly than the visible matter. This implies that a galaxy is surrounded by a cloud of dark matter that is a bit larger than the part of the galaxy that we can see.
Also read: The Equivalence Principle & Black Holes
The actual nature of dark matter indeterminate, but it is not composed of ordinary atoms or elementary particles, which interact with photons as described by the standard model. Dark matter is not a collection of small black holes or a group of little planet-like objects. Note that, general relativity predicts that light ‘falls’ in a gravitational field and because of this, light rays are bent toward massive objects. This means that massive objects actually act like lenses and focus the light from objects shining behind them. When such a ‘gravitational lens’ passes in front of a star, the star appears to get brighter. By observing a large number of stars and noticing how often they happen to brighten in this way, astronomers can ‘count’ the number of gravitational lenses out there. There are too fee such events for all the dark matter to be clumped together in black holes or small planets. Instead, most of it must be spread out more evenly. It is most likely that, dark matter consists of stable, massive, slow-moving exotic particles created at high energy in the first moments of the Big Bang and predicated by grand unified theories like Supersymmetry. The neutralino is the most likely dark matter candidate predicated by Supersymmetry. The neutralino is the lightest supersymmetric particle, thus the most stable, and has no electric charge, so it is not affected by photons and only weakly interacts with ordinary matter. Unfortunately, this makes the detection of neutralinos challenging, and no conclusive detection of dark matter in earth-borne detectors has been reported so far.
Also read: What is a Black Hole?
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