Such is the title of a lengthy article in the June 2001 issue of Astronomy. It is always dangerous to employ superlatives; "greatest" is particularly hazardous. Anyway, it is useful to review what mainstream astronomers consider to be their major unsolved problems. Naturally, we shall add a few that we think should have been on the list.
How multidimensional is the universe? For example, gravitons, which are believed to exist in a fifth dimension, are supposed to transmit gravitational force. This dimension is barely separated from our well-known four. The thin barrier separating us from the graviton universe seems to leak a bit therebyallowing gravity, the weakest of all our universe's forces, to exist. Sounds pretty far-out, but not as bizarre as string theory which requires many more dimensions!
How did the universe begin? The cosmic microwave background is much too smooth. If it was smoothed out by a sudden expansion of the universe (so-called "inflation"), what caused the inflation?
Why does matter fill the universe? in other words, where is all the antimatter that we think must have been created in equal amounts? (This equality is a human philosophical requirement. The universe can do anything it wants!)
How did galaxies form?
What is cold dark matter? This "substance" seems to be filaments threading the surfaces of cosmic bubbles (voids). It seems to be slow-moving and cold (no electromagnetic radiation), but no one really knows what it is. Apparently, it constitutes 30% of that part of the universe that we have so far detected. (We are doing a lot of guessing here!)
Are all the baryons assembled in galaxies? Baryonic matter includes protons, neutrons, and electrons. Baryons should be abundant in intergalactic space, but they are nowhere to be found.
What is the dark energy? Whatever it is, scientists have so far only been able to name it. It is thought to be associated with a repulsive force that is accelerating the expansion of the universe.
What is the destiny of the universe? Will entropy do us in?
(Sincell, Mark; "The 8 Greatest Mysteries of Cosmology," Astronomy, 29:46, June 2001.)
Comments, It is easy to add to the above list: (1) Why does anything exist? (2) Is there life elsewhere in the universe?
(3) Are redshifts really good yardsticks?
(4) Are there other universes, as distinct from the other dimensions mentioned above?