What are your thoughts about parts of our universe that we cannot yet perceive or sense?

There are many theories. What are some that have caught your attention, and then your perception/thoughts about them. Why do you think the theory(-ies) are valid, and what weaknesses do they have?

I personally think of dark matter like astrocytes in the brain – holding together the rest of the cells – or in universe’s terms planets and matter.

Keep this on topic please, long answers will get lurve by default.