[Pharmakon]

The existence of dark matter in galactic halos is inferred from the velocity dispersion of stars in a galaxy (sigma). It’s important to realize that this is a very rough kind of estimate because every aspect of it (as in the case of the M-sigma relation) is roughly estimated. Virial mass is inferred from average temperatures of gas by way of detexting X-rays. Central black hole mass is inferred through the M-sigma relation, and stellar masses are inferred through luminosity. Every so often a new paper comes out revising the overall estimates, but the proportion of DM to luminous matter stays pretty constant.

Now, in a sense this makes a “selfie” easier, because you just need a series of snapshots which help you make all of these inferences. On the other hand because we’re in the disk of our galaxy it is difficult to image because we have to deal with all of that hot interstellar gas and dust, and the extremely bright galactic core. I’m not sure how much, when you take into account a long period of time and many wavelengths can be observed, but I’d say we can observe the majority of our galaxy in at least one wavelength. Still, we’re not talking about “selfies” here, just really well educated guesses based on observations of our own galaxy and similar spiral galaxies.

So as far as inferring the existence of a dark matter halo in our own galaxy we’re pretty certain, and it seems to make up about 95% of the galactic mass. It’s based on something a lot more tenuous and complex than a “selfie” though. Once you’ve done your best to estimate the total luminous mass of the galaxy and it’s rough distribution though, and ruled out MACHOs with lending surveys all that’s left in the remaining narrow window is dark matter. The only other theory with any chance at all is MOND, but it has to thread crushingly narrow passages between observation and tests of GR.

[nonbel]

>"[dark matter] seems to make up about 95% of the galactic mass...crushingly narrow passages between observation and tests of GR"

GR only matches observation because you added in 20x more stuff that is undetectable other than as a deviation from the predictions of GR.

[Pharmakon]

That’s true, but it’s also true that hypothesis is preferred because of the abject failure or other theories and hypotheses to match those observations. The window for something other than DM is exceedingly narrow and only MOND has a hope of fitting through, and currently not in a form that’s very compelling.

[nonbel]

The point is this "window" isn't narrow at all once you allow dark matter.

Practically any theory plus 95% "dark matter" can match the data.

[PopeDotNinja]

On the tangentially related topic of dark matter + black holes, here's a YouTube video I found interesting on topic. It poses the question "do black holes contain dark matter?"

https://youtu.be/9Qis5VDOd18

The TL;DR is that dark matter can fall into black holes, but black holes don't hoover up nearly as much dark matter as regular matter.