William Leith in The Telegraph:
Why, Brooks asks, can’t physicists find a theory to explain how the universe works? Well, the universe contains particles, and these particles are guided by forces. The trouble is that the experts don’t really understand most of the forces and particles. “Almost all the universe is missing,” says Brooks. “Ninety-six per cent, to put a number on it.” Brooks surmises that there must be hugely powerful forces we don’t know about – or, to use the scientific term, “dark matter”. We know this – or, at least, we think we know this – because we don’t understand how gravity works.
Don’t we? Not really. In our solar system, we know that the Earth travels around the Sun faster than, say, Neptune for a simple reason: the Earth is closer, and is therefore subject to stronger gravitational force. But look at galaxies a little further out, and the same thing does not happen. In the Coma cluster of galaxies, objects at the edge are moving faster than they should. That must be because they are being held in place by something. Dark matter, almost certainly. Which might be just another way of saying, “we don’t know”.
Adding to the list of uncertainty, Brooks asks another question: what is life? Again, scientists don’t know. There are inanimate objects. And then there are living things. “But no scientist on Earth can tell you where the fundamental difference between these two states lies.” One definition might be that living things reproduce themselves. But then, so do some non-living things, such as computer viruses. And some living things, such as mules, do not.