What is the hardest question about the universe that scientists could not answer?

Here is a short list:

1....What does the universe look like beyond the limits to our visible universe? To answer this question, we would have to be able to 'see' to distances farther than light has been able to travel since the Big Bang. Big Bang theory and general relativity say that their is a lot more space beyond the 15 billion light years horizon limit, but we will never be able to see if the same kinds of galaxies as those around us exist 'out there'.

2...What were the conditions in the universe like at a time less than one second after the Big Bang?. Right now, we can use the COBE and WMAP data to examine how lumpy the universe was at about 300,000 years after the Big Bang, but we will not be able to directly explore what the universe was like earlier than about one second until we can use neutrinos, and they are very hard to detect in large enough numbers to form an 'image'.

3...What was the state of the universe like 'prior' to the Big Bang? The only known way to probe these conditions is to have at hand a 'Theory of Everything' that combines gravity with the rest of the forces we know about into a single mathematically rigorous description. To test this theory to make certain it is consistent with the way the physical world operates, we need to perform experiments at energies trillions of times higher than any technology can now imagine providing under laboratory conditions. Even so, we cannot really directly explore the cosmological conditions except by trying to re-create them in the laboratory, and from this assuming that the Big Bang worked the same way.