present state.
Several other major puzzles remain in physics. One is that current theories of
gravity (both Newtonian and Einsteinian) predict that a particular graph should
describe the rotation of stars in a galaxy—stars orbit the center of galaxies in much
the same way that planets orbit the centers of solar systems—but when one actually
observes the rotation of stars, the graph one draws is substantially different. It
appears that a great deal of mass should be in each galaxy, but none can be seen.
This missing mass is called dark matter, because it is matter that cannot be seen
(i.e., is dark). There are many guesses about the nature of dark matter, but no
definitive explanation has been found.
Similarly, when one uses general relativity to describe the expansion of the
universe and inputs what we know about the current state of the universe, it appears
that the expansion of the universe should be decreasing (distant galaxies should be
redshifted by smaller and smaller amounts). However, observing the actual
expansion of the universe reveals that it is in fact increasing. The source of the
energy that could cause this, called dark energy (by analogy with dark matter), is
unknown. Investigations of black holes, dark matter, and dark energy are ongoing in
physics, along with many other studies of many other phenomena, and the search for
a more complete picture of the universe continues.