Two points:

First, this question came up when a co-worker asked me to explain what the name of one of my workstations, anmut, meant. I told him it's a German word that means "grace, harmony in movement," among other things. He replied that it doesn't sound like it should mean that. He thought the word sounded, to an ear accustomed to English, too clunky, definitely ungraceful. He asked what the word is for something that doesn't sound like what it means, thus condemning me to this search for an antonym to onomatopoeia...

Second, your question about whether everything must have an antonym raises some really interesting (to me) points from philosophical and logical angles. Jumping immediately to mind are concepts like yin and yang, night and day, the binomial theorem, matter and antimatter. Is it possible for something to exist without some complementary non-existence? Is zero the opposite of one, and if so, is zero also the opposite of two? If negative one is the opposite of one, then where does zero fit in? What is the opposite of zero?

Whew! To quote Mr. Gumby, "My brain hurts!" Time to go hug the kids.