In article ,
writes:

I'm unfamiliar with this technique - microlensing refers
to the occlusion of distant bright objects, by nearer objects?
Thus gravitational lensing effects?

Yes. Not necessary occlusion, though; it's enough if a background
object is near the line of sight to a foreground object. Due to proper
motion, the proximity changes with time, and since the amplification
depends on the radial distance, one sees a typical brightening then
dimming of the background object.

[[Mod. note -- To the original poster (and anyone else unfamiliar
with the technique): The Wikipedia article
https://en.wikipedia.org/wiki/Gravit...l_microlensing
is an excellent introduction.
-- jt]]

I don't understand the primacy of the masses.
Wouldn't the statistics depend on the volume of
the 'dark' objects? That is, their solid angle arc,
how much of the sky they cover?

Not so much their own solid angle, but rather the solid angle within
which an appreciable gravitational-lensing effect occurs.

I don't find the reasoning compelling. You looked
at quasar variability, and concluded that MACHO
doesn't explain it. Isn't it a big leap to say such
objects don't exist at all?

The conclusion is not that they don't exist at all, but rather that
they cannot explain most of the long-term variability of quasars.
This contradicts a claim that most long-term variability of quasars
is due to microlensing, which in turn would imply that they make
up at least most of the dark matter in the universe. Executive
summary: Yes, at first glance long-term quasar variability is
compatible with microlensing, if one looks at individual light
curves. However, this hypothesis makes predictions about the
distribution of amplifications which are in conflict with
observations.

Also, big-bang nucleosynthesis tells us what fraction of the universe
is in baryons; there is no way that stars, being baryonic, could make
up a significant fraction of dark matter.

Seeing that 80% of the mass of the mass is 'missing', of
unknown character, all such origin theories are suspect.

Why? If you have a theory which predicts what all the matter in
the universe is composed of, let us know. Otherwise, we discover
it, component by component. We know more about some components
than about others. For example, we know how many baryons there can
be at most. This is not enough to explain most of the dark matter,
so there must be some other component. What is surprising or suspect
about that?

If one thinks that dark matter is somehow strange, one assumes, for
no good reason, that all components of the mass of the universe
must be detectable via our senses, or by astronomical techniques
at the stage they were during the second half of the twentieth
century.