“What is up, dog?”
Side note: the opposite of cat isn’t actually dog. The opposite of cat is goldfish. A goldfish loves water, is not fluffy, doesn’t exterminate every bird for sport, will never knock your favorite coffee mug off your desk, not bipolar, and I’m not allergic to them.
The opposite of cat is antimatter cat. When they come together, you have an explosion 18,000 times larger than Nagasaki.
The absence of a cat implies nothing is there. So the opposite of a cat is nothing (specifically nothing where a cat should have been).
So, another acceptable answer would be “what is up? Nothing.”
The absence of a cat implies the absence of a cat. There could be a dog instead or a Gasplanet which has a species of conscious whales wearing silly hats rolling in the mud to communicate the need for a fruity beverage. It could be anything, it could be nothing. It is not possible, from the given facts, to come to any concrete conclusion besides that a cat, is in fact, absent.
will never knock your favorite coffee mug off your desk,
I once shared a house with someone who had a fish that would thrash around in its tank so violently it would knock things off the cabinet it was on and get water all over the floor (I had no involvement in the care of the fish, but I think its tank was too small).
Not a goldfish, but still…
How do you know you’re not allergic? Have you ever rubbed one over your most delicate parts?