 Our universe is a mystery, it's a collection of mysteries. The more we learn, the more we realize that there's a lot more that we do not know. Here are five big mysteries. There is no reason to believe that our universe is unique. It is quite possible that just as our universe was created, many universes were also created. If that is the case, then we live not in a single universe, but in a vast collection of universes that physicists refer to as the multiverse. We can think of those as island universes, each with its own quirks, maybe even each with its own laws of physics. For example, there could be one of these other universes where gravity is a hundred times stronger than it is in our universe. If we could somehow prove conclusively the existence of multiple universes, of the multiverse, that would be a genuine intellectual revolution, a complete change in our cosmic thinking. The dark side of the universe is a really big mystery. The dark matter and the dark energy make up no less than 96% of what the universe contains. The stuff that you and I, the sun, the earth, the stars, the galaxies are made of, that is just the icing on the cake. It makes up only 4% of what a universe contains. The most likely explanation for the dark matter is that it's made up of some exotic fundamental particle that came into existence just a tiny fraction of a second after our universe was formed in the Big Bang. But it is a mystery. We have compelling evidence that it's there. We suspect it's a fundamental particle, but frustratingly, it has yet to be discovered. I think most people have heard about black holes. Fewer may have heard about white holes. White holes are in a way the opposite of a black hole. Black holes eat everything. White holes kind of vomit everything out. Nothing can come out of a black hole. Nothing can go into a white hole. Whether white holes exist or not, matter of heated debate. Of course, if they do exist, they are fascinating. And they may even raise the possibility that they act as tunnels to other universes or indeed to other epochs in our own universe. But they remain hypothetical objects. The current view in cosmology is that everything we see in the universe, everything started life as something called a quantum fluctuation. Sounds mysterious. Let me explain what this is. When the universe was born, it expanded very rapidly in a very short period of time known as cosmic inflation. As the universe expanded, these quantum fluctuations were stretched and the stretching seeded the universe with tiny ripples. These ripples yet are the origin of galaxies, galaxy clusters, planets, the earth and people. The mystery here is a really possible that a submacroscopic entity like a quantum fluctuation could eventually grow into the Milky Way galaxies. If you had told me that a few years ago, I would have thought but you read out of your mind. And yet today, the standard model of the formation of cosmic structure is exactly what I just described. Fascinates, not just physicists, fascinates everyone. And yet it is a relative concept. For example, if you travel faster and faster and faster, time slows down more and more and more. If you travel at the speed of light, time simply stops. If you look at the equations of physics, it's striking that time can go forward or backwards. The equations of physics don't distinguish. Because of this, many people believe that you can travel into the past because there's nothing in the equations of physics that forbids you doing that. It is quite possible that we could in principle travel back in time so long as we do not introduce any inconsistency with the future. So that's five big cosmic mysteries. But each mystery, as is always the case in science, leads to another even bigger mystery. And we haven't even got to possibly the biggest mystery of them all. Are we alone in the universe? But that's a whole new story.