could there be whole other galaxies and solar systems and being made of dark matter

No, Dark matter doesn't interact in the ways it would need to in order to form structures like planets, stars etc.

There are some other schools of thought on this, I have communicated with Foot about mirror matter and possible ways to detect it.

 

https://www.technologyreview.com/s/418687/first-evidence-that-mirror-matter-may-fill-the-universe/

 

https://en.wikipedia.org/wiki/Mirror_matter

 

I am kinda surprised to see mirror matter suggested as a solution to the dark matter problem. It would have to be at the weak interaction limit
Foot first proposed. In my conversations with Foot he seemed to think it might be somewhat more interacting than what Dark Matter seemed to be.

The trouble with that is the distribution of dark matter required does not match that of matter that can form stars, planets, etc.