The argument is this: The universe is extremely large—tens of billions of light-years across. The Bible tells us that made the universe a mere 6000 years ago. How does the light from distant galaxies reach earth then? Surely that size of the universe demands an enormous timescale?
But couldn’t God just have created the light in transit. No, that would be deceptive. It would mean a stream of light carrying false information for billions of years to come.
But surely even you must admit that the simplest and even most logical explanation (if you don’t first presuppose that the biblical account in Genesis is correct) is that the light, from galaxies millions of light-years away, did indeed travel for millions of years to get here?