It's most likely DNA dense regions of the nucleus, which are areas on chromatin that are not active.

The fact that you are not seeing any staining in cytosol and extracellular regions also support that it is not likely contaminated.

Normally you would see the contamination in the cytoplasm, so probably it is not a mycoplasmal contamination.

However, if you want to be sure (and you should do this check from time to time), use mycoplasma detection kits on your cells.

What's you cells?

It seems no problem. But as Alexandra said, you'd better to detect by kit or PCR.

What kits have you had the best luck with?  We have been using one from ATCC, but it has been backordered several times now, so we are looking for something more readily available...