Look at a galaxy, what do you see? Probably lots of stars. Nebulae too. And that’s probably it. A whole bunch of stars and gas in a variety of colorful assortments; a delight to the eye. And buried among those stars, if you looked carefully enough, you might find planets, black holes, white dwarves, asteroids, and all sorts of assorted chunky odds and ends. The usual galactic milieu.
What you wouldn’t see is what most of that galaxy is really made of. You wouldn’t see the invisible, the hidden. You wouldn’t see the bulk of that galactic mass. You wouldn’t see the dark matter.