May be you can show him some interesting videos of which as easy to understand for kids of his age! You can find some animated videos as well where such concepts are explained in an interesting way. Topics can include universe, big bang, space, earth, natural element, etc.
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The best way is to answer scientifically. Your 5 year old is old enough to understand the big bang and creation of the universe. It will be easier for you to explain because then, you would have all the answers!