The Gaia satellite is going to take a census
of 1 per cent of the stars in the Galaxy.
Which at first sight doesn't seem very much.
But, on closer examination, 1 per cent of
the Galaxy is a billion stars.
Which is a lot more than a six thousand that
we can see with a naked eye.
And Gaia in not just going to be counting
them.
It will be measuring the Doppler shift to
check their speeds, mapping their positions,
measuring their distances from us using a
technique called parallax, taking their temperatures
and working out what they're made of.
So from now on, when when we study the
Galaxy, it will be like when someone incredibly
short-sighted putting on a pair of glasses
for the first time - and 3D glasses at that.
And, once we know the speeds and distances
of the stars, we can tell the difference between
how much we think the Galaxy should weigh
and how much it actually does weigh.
And, therefore, how much other stuff must
be there as well.
Whether it's dark matter or something else.
