Question

Inspired by How do we know that dark matter is dark? and What is the temperature of the surface and core of a neutron star formed 12 billion years ago now equal to?

Answer 1

There were extensive searches in the early and mid 1990's for astronomical micro-lensing events, and these concluded that either the Milky Way is anomalous or there just aren't enough MACHOs to be a big part of the missing mass.

MACHO? That's MAssive Compact Halo Object. Anything small, heavy, cool and gravitationally bound to a galaxy. Cool neutron stars would count.

Micro-lensing? If a MACHO passes very precisely between a telescope and a distant star there is a detectable change in the intensity of the light from the star due to gravitational lensing, and that change follows a predictable curve depending on the intervening mass and the impact parameter.

So, with a sensitive (you need to be able to image down to 20th magnitude or better to get much data), computerized telescope you watch a nearby galaxy (say the Magellanic clouds) every night and count. With enough data you can make a pretty good estimate of the total mass of these things in the galaxy.

Aside: When I was an undergrad I worked on a small, cooled CCD telescope for a physics prof. And we'd try to image micro-lensing events reported by IAU telegrams. We were struggling with the tracking software, so it was a struggle to get imaging below 17th magnitude, but we did have a good run where we got a light curve that matched the big boys pretty well.