Maybe the sheer size of the universe? The speed at which light travels? The size of a quark? Out of all the things in the universe, what is hardest for you to possibly imagine, as long as it's generally accepted it doesn't have to be proven. For me it's both the size of the universe and the size of a quark. I mean, sitting here trying to wrap my head around how something can be so unbelievably large, yet also thinking how something can be so unbelievably tiny. Kind of ironic a little bit, how something like a solar system is similar to an atom even though their sizes vary beyond belief.