William Keel <keel@bildad.astr.ua.edu>

Dark matter is matter that is detected by its gravitational effect on
other matter rather than because of its electromagnetic radiation
(i.e., light).  This might be because of one of two reasons: 1. The
matter may emit light, but the light is so faint that we cannot detect
it; an example of this kind of matter is interstellar planets.  2.
The matter might not interact with light at all; an example of this
kind of matter is neutrinos.

The first astronomical instances of "dark matter" were probably the
white dwarf Sirius B and the planet Neptune.  The existence of both
objects was inferred by their gravitational effects on a nearby object
(Sirius A and the planet Uranus, respectively) before they were seen
directly.