I haven't found any evidence of this, but that's to be expected... How would that evidence look like even.

But there has always been rumors of this. And since there is money involved... I think it's not unlikely.

The argument against this is always "there are so many viruses, antivirus companies can't create them all". And nobody is saying that. It's enough to create one high profile virus and you will sell your anti virus software very easily.

But I think it was more common in the past. Today all companies will buy anti virus software, so it's guaranteed sales.