Yeah, let me answer this one.
"Astrophysicists are (i) _________ the fact that the Milky Way is a totally unremarkable galaxy, since this very ordinariness affords them a template with which to make viable extrapolations concerning the structure and dynamics of many other galaxies—both known and unknown."
So they're actually pretty happy about the fact that it's unremarkable. This unremarkability means that they can figure out other galaxies, an opportunity they are now "afforded" to do (they couldn't before).
They're happy. Or, as one might say, it makes their hearts happy (they're
heartened).
Surprised doesn't work here, as "since" means because. If they were surprised, we'd want something like "since they expected it to actually be remarkable."
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