Where is the Dark Matter?

"N" == Notorius writes:

N How exactly can anybody be "right" about a theory that hasn't been
N sufficiently proven yet such as "Dark Matter" AKA "Yet Another
N Fudge Factor Theory" akin to Einstein's "Cosmological Constant"?

For the record, dark matter is not a theory. Dark matter is a
hypothesis to account for various discrepancies between theory and
observation.

If one assumes that general relativity is the correct theory to
describe gravity[1], then various observations indicate that there is
far more matter around than we detect via emission of photons. So one
has two options:

1. There is dark matter, matter that is either too faint for us to
detect or in some cases that is hypothesized not to emit photons at
all.

2. General relativity is not an accurate description of gravity.

Most mainstream astronomers and physicists pick #1, for a couple of
reasons. First, GR has passed all of the other experimental tests
thrown and it, and, second, we know examples of things that are
difficult to detect at large distances (e.g., planets) or that do not
emit photons at all (e.g., neutrinos). Some physicists have rejected
#1 and tried to come up with alternate theories for gravity (e.g.,
Modified Newtonian Dynamics, MOND).

[1] On the spatial scales of interest to dark matter, the fact that GR
and quantum mechanics appear difficult to reconcile is not important.

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