What are the large round dark "holes" in this NASA Hubble image of the Crab Nebula? I came across this image of the Crab Nebula taken from NASA Hubble telescope. What are the large round "holes" and how are they formed?
I think my deleted answer to your previous question covers this well, so I'll add it here.
These two spots are known as the east and west bays of the Crab Nebula. They appear to be the result of a torus partially encircling a section of the nebula. The pulsar's magnetic field interacts with the gas and dust in the torus, which blocks synchrotron radiation from gases in the nebula. It has been conjectured that the torus is the result of a $\sim0.1M_{\odot}$ ejection of material from a circumstellar disk prior to the supernova, based on studies of the proper motion of gas on the edges of both bays.
References

Fesen et al. (1992)
Hester et al. (1995)

We can also rule out that this is the result of a defect in Hubble, because these features have been observed in different wavelengths, including infrared, visible, ultraviolet, and x-ray (though not in radio or gamma-ray wavelengths):


Image courtesy of Wikipedia user Hunster under the Creative Commons Attribution-Share Alike 3.0 Unported license.
The infrared, ultraviolet, and x-ray images come from the Spitzer Space Telescope, the SWIFT observatory, and the Chandra observatory, respectively.