The right answer is dark matter.
The galaxies of our Universe seem to achieve a real feat. They turn so fast that the gravity produced by the observable matter they contain can not hold them together; logically, they should have been discarded for a long time. The same goes for clusters of galaxies. This is why scientists are convinced that an invisible element is involved: something that we have not yet been able to detect directly gives these galaxies an extra mass, which produces the extra gravity they need not to do. This mysterious presence is called "dark matter".
Unlike ordinary matter, dark matter is not sensitive to electromagnetic force. As a result, it can not absorb, reflect or emit light, making it extremely difficult to detect. Scientists have been able to deduce the existence of dark matter only from the gravitational effect that it seems to have on the visible matter.
