Islam IS the true religion of humanity.

[Ksa]

Um ksa, the universe is 13.8 billion years old, the earth is about 4.5B. Other then that your argument is solid.

How do we know the age of the universe? How do we know it wasn't always here?

This Universe was not always here, it has a beginning and it has an end, but what gave birth to the universe itself has no beginning and no end. It's an ongoing process according to Brian Greene, an infinity of universes pop up continuously, each having a different coefficient of dark energy or energy of empty space. It's called the inflation theory.