The Big Bang?

In Astonomy we just went of the Big Bang theory. There are a few things I don't understand. For it to be a theory means it isn't nec. true. Yet they teach it as truth? Something else, for everything they can't explain they make up something so that they can "connect the dots". For example. In galaxies stars orbit around a central point. In our galaxy, the milky way, we everything orbits around the sun. Part of the problem is, the fact that the stars and our earth for that matter don't just fly off into space. True, there is gravity but not enough. This is the case for everything out in space and for every galaxy. So scientists have labeled the un-seeable force as "dark matter" *twilight music in the background* So now, scientists use that (dark matter) as the force that keeps all the stars in orbit etc....does that not sound a little weird and more like a made up name and concept? Something else that is weird...the inner planets orbit at the same speed as the ones on the outer spectrum. Normal physics would tell you that the inner ones would spin faster....hmmm...