Grandma's House

It seems our Milky Way has a little buddy. A sort-of companion galaxy/globular cluster just beside the MWay. Which brings me to my latest rant: How do they know what's out there? They've never been there. Take this paragraph for example:

If it is a galaxy, then it ought to contain a bunch of dark matter, mysterious stuff that can't be seen but that contributes more mass to galaxies than the collective heft of stars, gas, planets and dust. Problem is, nobody knows how to detect dark matter directly. It's only suspected because without it, galaxies don't have enough mass to hold together as they do.

Okay, so dark matter can't be seen, and they have no way to detect it, but we're supposed to believe it's there and they know all there is to know about it. But they seem to have a good reason to believe it's there. If it wasn't there, what would hold the galaxy together? Please! Until they have evidence, real evidence, as far as I'm concerned they're just extrapolating just to hear themselves talk. A little proof would go a long way.