In calculating the Milky Way's mass, based on its rotation, astronomers find there should be more mass: they call the deficit "the missing dark matter".
The matter contained in stars and black holes makes up most of the known mass in the universe, possibly all of it, but nobody is sure at this stage.
In 1783, John Michell outlined the nature of a Newtonian black hole, based on the idea that light consists of particles, and so would be affected by gravity.
In 1916, Karl Schwarzschild offered a singular static solution of gravitational field equations which describes a minimal black hole, and sent it to Einstein.
In 1916, while fighting on the Russian front, Karl Schwarzschild applied relativity to the inner workings of a star, then died of a battlefield illness.
Black holes are so massive that no light can escape past a limit of the black hole, a surface called the event horizon. This is not the edge of the black hole.
A black hole is typically surrounded by an accretion disc, a fast-rotating disc of dust and other material which eventually falls down into the black hole.
Schwarzschild concluded that when a star contracts under gravity, there is a point when the gravitational field is so huge, nothing, not even light, can escape.
The radius of a star of any given mass when it reaches the stage of collapse at which it traps all light is now known as the Schwarzschild radius.
Our Sun has a Schwarzschild radius of 2.5 km, and if it ever actually shrinks below that radius, will become a black hole, from which nothing can escape.
In 1967, John Wheeler, introduced the dramatic term 'black hole' to describe a concept that went back to earlier times, but never with such a name.
In 1972, James Bardeen, Brandon Carter, and Stephen Hawking proposed four laws of black hole mechanics in analogy with the laws of thermodynamics.
As material falls into a black hole it loses electrons and is broken down into charged particles, and as these accelerate, they emit large amounts of radiation.
In 1973, Ostriker and Peebles found the amount of visible matter in typical spiral galaxies was not enough for Newtonian gravitation to keep the disks together.
Wormholes are theoretical structures associated with black holes and loved by science fiction writers as possible ways to cross large lumps of space, fast.