Gravitational lensing
There is no great chance of observing this phenomenon
-- Albert Einstein
-- the Hubble Space telescope
(among others)
What is it?
Here is a star-forming galaxy.
Here is a massive elliptical galaxy.
Here is a massive elliptical galaxy in front of a star-forming galaxy.
The elliptical galaxy is so massive that its gravity is curving and focusing the starburst galaxy towards us like a lens...
...a gravitational lens.
What is a galaxy made of?
Here is a model spiral galaxy.
It has a bright spherical core and a rotating disk of stars, gas and dust.
As it turns out, that's not the whole picture.
The whole thing is suspended inside a spherical halo which is completely invisible...
Fortunately, this halo of dark matter still has mass...
This means it still behaves as a lens, magnifying and warping any light behind it.
A galaxy is much more than its stars...
...and gravitational lensing is the best way we have of observing the invisible.
