I recently ran into this NASA Hubble image of the Crab Nebula in this article talking about its core. What I am interested to know though is: what are these round dark spots appearing in the image (upper right, lower center and lower left for example)? "Holes" in the Nebula? Objects in front of the Nebula, visible against its illuminated gas? Blemishes in Hubbles lens(es)?
CLARIFICATION: I am asking about the small pixel like dots. Not the large "holes" from this question.