All other Universes. As Dr. Kaku explains, we believe that our universe will one day burn out. All stars will cease to create heat and light. Black holes will eventually suck up all of the matter and dissipate. Once the black holes dissipate, they will leave nothing behind.
It is important to remember that this is all theoretical. But it is also very enlightening, and gives some extreme perspective on our current lives. When you truly understand the fate of the universe(and more than likely the fate of mankind) will eventually cease to exist, it is easy to understand the value of your life.
Because the value of your life is then placed into your own hands. Many ask the question: "What is the meaning of life? Why are we here?" The answer is that we will never truly know. We must place that importance within ourselves. Understanding that our Universe is already billions of years old, and will live for many billions more years(but not infinitely) makes the potentially hundred-ish years we live seem very small.
Why do I explain all of this? Well, it is in the hopes that those who read this will see the Universe as I do. In a deep understanding that our every day choices matter greatly to who we are, because only we can measure the value of them. You could die tomorrow and the Universe(and Multi-Verse) wouldn't care. It doesn't matter how valuable you were even to human society. In fact, humans could cease to exist tomorrow and the Universe still won't care.
But then the question must be asked: If there is nothing there to Observe our Universe... Does it really exist?