Well there is a theory, David, about what
dark matter is. 
You talk about different universes and let's
say that our universe is a sheet of paper. 
We live our entire life on this sheet of paper,
but directly above us there could be a parallel
universe, hovering right over us, perhaps
inches, centimeters away and objects in this
parallel universe would be invisible. 
Light travels beneath the universe, so we
never see this other galaxy.
But gravity, gravity goes between universes
because gravity is nothing but the bending
of space, so if the space between two sheets
of paper is bent slightly gravity then moves
across.
So think about it. 
This other galaxy in another universe would
be invisible, yet it would have mass. 
That's exactly what dark matter is. 
Dark matter is massive—it has gravity—but
it's invisible. 
It has no interactions with light or the electromagnetic
force, so there is a theory that says that
perhaps dark matter is nothing but matter,
ordinary matter in another dimension hovering
right above us. 
We should also point out, however, that there
are other theories too. 
Dark matter is the cutting edge of science. 
Some people think that maybe it is a higher
vibration of the string. 
All the atoms of our body represent the lowest
octave of a tiny rubber band vibrating all
over our body, and the rubber band could have
a higher octave. 
That next octave could be dark matter.
So that's yet another explanation for what
dark matter might be.
So the bottom line is this. 
There is a shelf full of Nobel Prizes waiting
for you, waiting for anyone who can come up
with a convincing and experimentally verified
explanation of the origin of dark matter.
