The universe is our vast, complex and mysterious home. Science has discovered a lot about it, including how it initially grew rapidly from the initial hot condensed state, particles formed out of the enormous energy and the fundamental forces devbeloped gradually. Later, atoms formed, and later still stars and galaxies, until it reached the state we see today.
But how it started remains a mystery.
It seems that either something caused it to begin, or nothing caused it but it began anyway, or it never began because it has always existed.
There are arguments for and against each view, but there are good reasons to think it hasn't always existed, and beginning without a cause goes against everything we know about the physics of the universe.
Concluding it had a cause seems reasonable, but this conclusion does open up further questions.