Time..

[Jackalope]

And if you hypothetically, were to sit in the middle of space, between galaxies, how would you measure time?

Likely the same way that we establish time references - by measuring radioactive decay.

but what if you passed close to a black hole?

Are you concerned with the passage of time in your own reference frame? No problem, continue using whatever method you use until if and when spaghettification occurs. Worried about someone else's reference frame? Rosta ruck with that.