Is God the uncaused creator of the universe or is the universe uncaused and eternal? Has the universe always existed? Before the 20th century there wasn't any way to prove which theory was correct. Einstein's equations for the general theory of relativity brought to light the fact that the universe was either expanding or contracting. That is why he came up with the "cosmological constant" in order to support the idea of an eternal universe. However, when Edwin Hubble discovered the "redshift" concept, which proved the universe was expanding, then the "cosmological constant" idea was abandoned.  (See the "Cosmological Constant problem"). 
One of the most reasonable explanations (in my opinion) as to why the universe could not be eternal is the second law of thermodynamics. That is the law of increasing entropy which states that everything is running down and out of energy. Since it has been proven that the universe is expanding, then logically, the universe must have had a beginning (the Big Bang) when all of the available energy and matter were compressed into a singularity. Someday all that matter and energy will get used up and that will be the end of the universe.
For a more in-depth explanation of this subject including the 5 proofs of why the universe is not eternal please see: Eternal Universe.
- "Is God Just a Human Invention?" Sean McDowell & Jonathan Morrow - Kregel Publications Chapter 5 "How Did the Universe Begin?" pages 72-73.
- Cosmological Constant Problem