Whether Universe expands or not, this thing definitely did ;)....
Basically, the Mad Scientist kept a perfectly normal half litre bag of milk in some remote corner of his kitchen and completely forgot about it. Four summer nights and four summer days it lay there, before it was discovered in the state you see above.
When the Mad Scientist discovered it, he thought, "Aha! Just in time!" That is, just in time for a Photo-op, and just in time to avoid a Nasty Surprise that would have gone 'Blooooofsh!' and forced him to clean up some yucky creamish creamy goop!
So what did the Mad Scientist do next?
1) He grabbed the photo-op!
He threw the budding and balooning model in the nearest public garbage dump. He actually waited two more days for the thing to go Blooooofsh! Unfortunately it never did. It shrunk back into a floppy lumpy little bag, even smaller than the original half litre bag of milk.
There are two hypotheses doing rounds in the scientific circles around this strange phenomenon:
1) Black Hole Theory:
The unprecedent occurance of bubbling white gloop enclosed in a extremely tight plastic boundaries led to the creation of a localised Black Hole. All the anti-dark matter got absorbed into it. But the disgusting taste of the stuff made the Black Hole faint. And further, owing to the soporific effects milk, it never woke up again. So everybody else lived happily ever after.
2) Mosquito Hole Theory:
Basically some mosquito sat on the bloated thing. And owing to the intense processed going on inside it, the rotund thing was quite warm. So when a passing mosquito sat on it, it though it had landed on somebody's butt and gave it a prick. (Yes mosquito's job has become very tough these days. Its very difficult to distinguish between a plastic bag of rotten milk. After all many humans are so plastic these days. eg. Aishwarya)
So what really happened?
It is expected that Universe started expanding starting from the Big Bang. But this particular Big Bang was expected to happen after the expansion.