What is the meaning of the statement ; "Physics is cumulative"​

As long as your math skills do not deteriorate in the intervening semesters, not really.

I may hold an unpopular view of this, but I am of the opinion that the algebra based E&M class is a waste of both your and your instructors time. That you would honestly get more from a short lecture electrostatics and magnetostatics (with the usual labs), on analogue and digital electronics, and then just skipping to geometric optics and (very simple) thermodynamics and spend the rest of the course on thermodynamics.

As I have been made to teach the course before, the optics and thermo are smashed onto the end of the course along with a little bit about ionizing radiation and oddly a tiny section of special relativity. Presented in this way, these subjects seem entirely disconnected to the student because they do not possess the mathematical tools to connect them (or at least we assume they do not so we do not present the material in a way that promotes this connection).

3. dark matter

empty space could also be a simple vacuum