Hello This is my first post here. Although I've often read posts in Physics Forums, I've never actually contributed anything - so here goes! I always enjoy a good hand-waving argument on orders of magnitude; when I first started my astrophysics degree, one homework question was "estimate how many piano tuners there are in this town" - that was it, no other clues! Of course, it was designed to get us to think of ways of solving a problem by making realistic guestimations. Another argument I admired goes that there cannot be an infinitie number of stars in the universe otherwise there would be an infinite amount of light in the sky, so we would not expereience darkness at night. Anyway, I was sitting in the library today (should have been working on a Solid State problem) and started wondering: if I took the energy of a photon as a "rest mass" and multiplied it by the number of photons in the universe would that be anywhere close to the estimated figure for dark matter? So I looked up some figures from not-very-reputable sources (this is just hand-waving, after all) and got this: Number of photons in the universe: 10^91 (source: Science Blog) "Rest Mass" of a photon: 4.2 * 10^-40 kg (source: Wikipedia) So, total estimated photon "Rest Mass" = 4.2 * 10^52 kg. Now compare to Dark Matter in the Universe: 3.5 * 10^52 kg (source: Wikipedia) They match within 20% - well within an order of magnitude! Well, I thought it was interesting even if doesn't mean anything! Anyone got any other hand-waving examples that they can think of?