17. Are we alone in the Universe?

Aliens are courtesy of The Jim Henson Company.
Will SETI (Search for Extra-Terrestrial Life) one day finally receive that long awaited telephone call? Will the Phoenix lander discover microbes beneath its microscope (albeit very tiny ones)? Will future craft find beings inhabiting the oceans of Europa that make whales look like shrimp? Our own galaxy contains roughly 100 billion (yes — 100 thousand million) stars. In addition, there are about 100 billion galaxies in our observable Universe. That's 10,000,000,000,000,000,000,000 stars (assuming our galaxy is average).
Considering the frequency with which we are discovering new planets, it seems more than possible that many planets are habitable and may harbor life. The question boils down to the likelihood of life making that first step from non-life, which is a complete unknown. But it is a question sure to be at the forefront of human thought and scientific curiosity. Perhaps we are already being visited. Scientific evidence is lacking, but it doesn't seem so unlikely to be impossible. See the Drake Equation to play with more astronomical numbers on alien life.
18. Is the Universe inherently deterministic or is there "true randomness" in nature?

Do steadfast laws underlie quantum physics? At the macro level, all physics seems deterministic; i.e. every action is causally linked and predictable in theory based on the events preceding it. Current quantum theory seems to indicate an inherent randomness in the behavior of quantum particles. Some claim that this is due to an incomplete understanding of nature — that there are hidden variables and even at the quantum level, causality holds true.
The question remains: is there "true randomness" inherent in nature at the subatomic levels? I have read that most physicists currently lean toward true randomness. If there is no "true randomness", then every event in existence was determined by those before it, thus eliminating the possibility of free will. However, if there is randomness, this at least leaves open the possibility of true free will.
Obviously, we are edging into philosophy here — and a topic which we could debate for years, no less. Nonetheless, if physicists can reconcile quantum physics with Newtonian physics and relativity, and all the other weird quantum stuff I am light years from understanding, perhaps they may answer the question of the nature of the existence.
19. What is the maximum carrying capacity of the Earth? Will we enact global population control measures?

Just how many people can live on the Earth? Some would argue that we have already surpassed the carrying capacity, while others believe we have a ways to go.
Given current birth rates and ever-expanding life spans, it seems inevitable that we will be forced to enact population controls on a world scale. It is science that will have to tell us exactly what our resources can handle. No doubt, technology can increase our carrying capacity, if utilized properly.
20. What is the ultimate fate of our Universe?

Will our observable Universe eventually cease in a frozen motionless entropic heat death? Or will the dark matter and energy pull all matter back into the singularity from which we exploded (The Big Crunch or Gnab Gib)?
This is still a hotly debated topic. We lack much crucial data. However, current measurements indicate that the Universal expansion is accelerating and not decreasing in its rate of expansion. How much dark matter is actually out there? And...
21. What is dark energy and dark matter, anyway?

The distribution of dark matter obtained from a large numerical simulation. Note how the dark matter is clustered into dark matter halos, which are connected by a large filamentary network.
I don't have much to say about dark matter or dark energy, and I'm not sure that physicists have much more. Actually I'm sure that they do — I am probably just avoiding them.
Something seems to be out there, swirling within galaxies, holding them together, and pulling groups of galaxies into clusters and superclusters. We have inferred its existence from its effect on other mass. More than that I cannot tell you. I hope that science will tell us much much more in the coming years.
22. Is time travel possible?

Yes. Forward at one second per second. I jest. Again, theoretical physicists have come up with scenarios in which some form of time travel might be possible. They all seem baffling to me.
I had high hopes for the Time Traveler Convention of 2005, but unfortunately it seems that humans will not eventually discover time travel, or that when they did, they will have never heard of the Convention and so failed to show up.
23. What is the true nature of existence? Parallel Universes, multiple dimensions, strings?

Superstrings may exist in 11 dimensions at once.
Physicists — I leave this one to you. I have tried on many occasions to wrap at least a few brain cells around string theory (may those neurons rest in peace). If science ever comes to grips with the nature of our physical reality and devises the Grand Unified Theory of everything, I sure hope the math can be translated into more conceptual terms.
If it turns out that we live in only one (or four) of 13 dimensions or some other such craziness, we prove it, and I still cannot understand it, it will be a sad and anticlimactic day.
Well, those are the best questions I have to offer. Again, please feel free to leave your own two cents. I am sure there are worlds of interesting and important scientific questions left to be answered.