Outside the UK I am not allowed to watch the show.

It has been eloquently explained to me that dark matter is like the twigs, branches, limbs and trunk of the tree of the universe. The leaves that cling to the tree are only the relatively light weight normal matter that we can see.

We can not see the wood and perhaps the wood can not see the leaves because the nature of space or the nature of matter does not allow it, but the tree stands.

The visible universe is composed of particles that are realized mathematically in whole numbers in their properties and spin but dark matter may be measured only in relative fractions.

That fractionalized particles are allowed to slip by and through our familiar matter and itself, leaving only gravity as a mutual interaction is a HUGE clue. Now all we need to know is what that means and by the way, learn what gravity really is.