What are the most mysterious unexplained phenomena in the universe?
Here is a list of some of the most mysterious unexplained phenomena in the universe:

*  Fast Radio Bursts: Mysterious and powerful pulses of radio energy that have been detected from distant galaxies.

*  Dark Matter: A mysterious form of matter that makes up around 84 percent of the universe but cannot be seen or measured.

*  Gamma-Ray Bursts: High-energy explosions that occur in distant galaxies, releasing more energy than the Sun does in its entire lifetime.

*  Cosmic Inflation: An accelerated expansion of the early universe that happened in a fraction of a second, but had a profound effect on the universe's structure today.

*  Dark Energy: A mysterious form of energy that is thought to make up around 68 percent of the universe and is responsible for its accelerating expansion.

*  Neutrino Oscillations: The mysterious ability of neutrinos to switch between different forms, which is still not completely understood.
Can you explain the process of quantum entanglement to me?
Two quantum systems are said to be entangled when a measurement on one gives information about the other. Usually this happens when the two systems have interacted in the past in some way. A common example is the entanglement of two electrons. Electrons have indeed a property called spin, which can assume value 1/2 (spin up) or -1/2 (spind down). Two electrons in the same position needs to have opposing spins, due to Pauli exclusion principle. So if you have two electrons in the same place, that are then separated, you get an entangled state. A measurement of the spin of one particle, immediately gives you the spin of the other.
Let me know if I've been clear!