Why Isn't the Universe a Necessary Being?

This is in response to Plantinga's type of ontological argument. I ran into one today on Facebook. If I grant that a necessary being is possible then God cannot not exist and the Christian theist wins the day, it seems. So here's a challenge. I guess I like issuing them. ;-)

Show me why the universe is not a necessary being. I'm not sure you can rule that out as quickly as you think. Philosophical arguments are okay. Only the hard evidence matters. The universe is nothing like anyone would expect prior to Darwin, Einstein or quantum mechanics. So why would we think a philosophical argument prior to the available evidence should hold any weight at all?