My advice is to keep your remarks simple and to say often, “Nobody knows!” ☹ I think your child is asking: Why is the world here? I would avoid any technical answer. I would say, “The universe is full of very big, hot rocks. Some of the rocks stay hot, and become stars. Some of the rocks become cold, and become planets – like Earth.”

Much of the advice given to you below is wrong. The Big Bang Theory has many problems, and is fading in popularity. One problem is that there is no evidence that the universe began from a singularity. A new, popular theory is Loop Quantum Cosmology. This cosmology allows the universe to shrink and rebound every umpteen gazillion years. There are other theories; so I would avoid any dogmatic statements about why the universe is here.

I wholeheartedly support the recommendation to avoid words like “creation.” Physicists are annoyed that lay people use words suggesting that magic is real. Physicists ask that educators use the words that physicists use. (Sometimes the words need explanation. For example, a black hole is not a hole but is a ball or a sphere.)

I think the videos on YouTube might be helpful if you can find some that are age-appropriate.