Dear Yousuf Khan:

On Monday, October 24, 2016 at 11:18:24 PM UTC-7, Yousuf Khan wrote:
....
Well, let's look at it practically. The Milky Way by
itself is 100,000 LY across on its disk, probably 1
million LY across on its halo. So neutrinos released
now from the various stars of the galaxy will remain
within the borders of the galaxy for between 100,000
to 1 million years.

Well, they'd ("they" = neutrinos) be *average* about half that since they are generated across the disk, and if they were generated here, they'd be very energetic. If they were created much nearer the Big Bang, and are still hanging around, say in orbit, their total energy will be very close to their rest energy.

Enough time to remain a part of the galaxy's mass.
Even photons will do the same. So there's a large
amount of time that energy will remain within the
borders of a galaxy, and even larger amount of time
that it'll remain within the borders of a galactic
cluster or supercluster. Now that we have detected
gravitational waves, there's another even more
humongous source of energy (converting several solar
masses into energy at a time) that we know travels
at only the speed of light.

Do we *know* that, however? Have we always been able to correlate a wave detection with a visible "flash" (or loss of a periodic source) in the proper direction? Side issue...

So lots of energy stays locked into regions just
due to the slow passage of time.

Yes. So this is true of Newton, General Relativity, and its quantum replacement. But it isn't much, if the sum of each flavor of neutrinos is only 18 eV.

So is this contribution 1%, 10% or the necessary 600%? I'd say it was more along the 1% lines...