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Why aren't these galaxies being "lensed" by the closer M101? Because detectable lensing is by far the exception, not the rule.
So what? How does that determine whether a more distant quasar can be seen through a galaxy. Even with perfectly aligned lensing being "the exception", in this case it provided a rare opportunity to show that a quasar is present behind a galaxy without being visible directly through the galaxy.
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Consider:
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"In fact,M101’s disk is so thin that the Hubble telescope easily sees many more distant galaxies lying behind it.Seeing these background galaxies shows that a galaxy’s disk is really mostly empty space."
This looks like little more than a red herring. The Hubble photos of the pinwheel galaxy are, as it states, of the thin disk. In contrast, the quasar aligned with NGC 7319 is aligned with the central core of that galaxy, which is much thicker and much more opaque, making it less likely the quasar is behind the galaxy