About 40 years ago the physicist Vera Rubin made a strange discovery: considering the weight (mass) of all stars belonging to the Andromeda Galaxy it turned too fast. In order to be clear, have a look at this merry-go-round. It turns at the right speed. But if it would turn ten times faster, the little chairs with the kids would first be horizontal and some seconds or minutes later the chains would break. Same for the outer stars of Andromeda. At the measured merry go round speed the galaxy should disintegrate!
The center of all those galaxies behaves in accordance with the laws of gravitation, see Isaac Newton and Albert Einstein. The problem is thus only with the outer portion who does not fit in.
To make it simple (that's my specialty) there are two solutions: Einstein's law of gravitation has to be modified - because only partially correct - or some extra weight has to be added to those galaxies so as to make them behave as they should.
Thus the theory of the black matter has been invented. And as time goes by, the theory of the black matter turns into a fact. Right now, the black invisible stuff is staple food for 99,9 percent of all astronomers. And it should be said that the black matter has one big advantage: no need to tamper with Albert Einstein's findings. And that's important because he is something like a God of Science.
Cast doubt on Holy Albert's theory? Forget it. Let's better gorge those galaxies with some extra weight - in fact about 95 percent, to be added to the 5 percent of conventional matter we know - and the scientists can avoid to rock the boat, to create fuss, avoid the shit to hit the fan, to be considered a lame brain half-wit.
That is the spiral galaxy M81. I suppose this one, too, turns too fast but what a beauty. And here, last not least, is our advent wreath, made by my wife to hang above the chimney till year's end. A home-made galaxy.