Expert: Tom Whiting - 10/12/2011

Question
Hi Tom, maybe this is more of a Physics Q, but I've liked your answers before so I'd like to give you a try!  Ok, so we hear that one of the biggest mysteries today in science is dark matter, what is it, where is it.   I heard Neildegrasse Tyson in a video clip say that the "missing" matter can NOT be in black holes bec "it's all been accounted for" (words to that effect).   I assume that to mean astronomers view and tally up visble phenomenom, black holes being center of galaxies or using other visual cues such as accretion discs/jets, etc.

So, here is the Q=   how about black holes that have consumed all nearby matter and are simply "free floating" in the vast stretches of empty space between galaxies and nebulae?  These would not be undetectable in any way yet may hold huge quantaties of matter, possibly even whole galaxies.  We know Andromeda and the Milky Way will collide eventually, and we see many remnants of collided galaxies,  but what about a galaxy somewhere in the universe that is relatively isolated, this galactic sized black hole eventually consumes all the material... there it is, in the middle of the darkness like some monster hiding.  This explanation to me seems so simple (too simple) yet it seems to answer the Q of dark matter?  What am I missing?

Thanks
Tim

Answer
Hi Tim,
Even if you allow for unknown "free-floating" black holes, and unknown very massive black holes,
you'll still be far short of the required mass/energy.  So you might take the baryonic (normal) matter up from 4 to 5%, but there is still a huge deficiency which dark matter (and more so, dark energy) has to account for.
So Tyson is correct, and we already know that the dark matter component is somewhere around 23%, so that leaves dark energy to account for a whopping 72 or 73% of the matter/energy of the entire Universe.
So there are really 2 big mysteries... dark matter AND dark energy, and we have no idea what they are.  (The word 'dark' just means that we haven't a clue).  We do know that dark matter is "something".. WIMPS? (weakly interacting massive particles) MACHOS?  (Massive compact halo Objects).
But dark energy seems to be just an inherent part of all space (which doesn't seem to weaken with time or distance)... an intrinsic part of all the space-time continuum that we just don't understand as yet. (Causing the Universe, the space - not the matter... to accelerate it's cosmic expansion).  In other words, dark energy is all around us... we just haven't (or can't because it's so weak nearby us) detected it locally as yet.  Whereas dark matter may not exist close by us, or if it does, we just haven't invented the right detectors as yet.  That's the best we can do right now in the very beginning of the 21st century.  Wait another 90 years.
Hope this helps,
Clear Skies,
Tom
Erie, PA