What is your frame of reference if you are standing on the sidewalk and see the bus go by? how can you tell that the bus is moving?

Explanation:

When you are standing on the sidewalk and see the bus go by. You are in a frame of reference of sidewalk, trees and buildings of surrounding. The bus is in a state of motion. So, its in different frame of reference.

Let's say some part of a sidewalk and buildings are visible for you before the bus id passing by. Now as bus is moving it may come in path of the visible objects available for you.

So, the moving bus comes in the path of visible objects. Thus, this conclude you that bus is in motion.

3. dark matter

empty space could also be a simple vacuum