To describe an objects position, you must first choose a reference point as a starting place. from there you must specify the

The question is not full.

But we must then specify in which direction do we want to be positive.

Explanation:

If we have to object moving up or down, object A and object B respectively.

If we choose up as negative, then object A is moving towards the negative position, if it is displaced by 21m, then is 21m in the negative direction.

The statement is the same for object, with the change in direction, in this case, it will be positive.  

3. dark matter

empty space could also be a simple vacuum