If our galaxy (for example) consisted of just the stuff we can see or otherwise
observe/measure - stuff you could call 'light matter' - it's thought it would not
stay together; the stars would drift off because the mass that we are aware of only
accounts for perhaps a tenth of the gravity needed to bind things where they are.
'Dark matter' is the term we apply to objects that we have been so far unable to
detect, but that we still reckon exist, because it would account for certain
phenomena that we have observed, such as the fact that stars do tend to cluster
We don't know what form this dark matter takes, what kinds of particles it consists
of. We don't know the precise location of any. That, unfortunately, is the whole
point about it being 'dark'. We don't even for certain that it is there at all. I
suppose it's always possible that there could be some hereto undiscovered force or
forces at work in our universe somehow supplementing the effect that gravity has.
So I think it may be a little optimistic at this stage to try and explain where it
comes from.. However, there are no shortage of theories about dark matter...