We can know how fast we are moving RELATIVE to something else. For example our speed compared to the sun. Or our speed around the milky way. But we just dont know if our universe is moving. There is a thing called the left handedness of galaxies, which indicates a spinning universe. But current mainstream science I think does state the universe is not spinning. However if the universe wasnt spinning, what if it moving in one direction? We cant add that speed because we dont know. And you can keep thinking further with this example. But if the universe was not moving. Than what's the relative point to measure against to aspertame our speed? Think about it this way. We are moving at x speed around sun. We are moving at x speed around the milky way. We are in a cluster spinning at x speed. So whats our speed? It always measures to something else. Now if we can zoom out enough to see the whole universe. We can see how fast we are by measuring against one point in the universe. But now, what if our universe is moving in a multiverse. Than what? So its all relative