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The white fog or smoke that appears as a champagne bottle is opened is condensed water from the atmosphere. It results from the pressure/volume work associated with the change in pressure between the sealed bottle (approx. 5 atm) and the open bottle. The associated cooling is sufficient to condense or even freeze water from the surroundings.

For thermodynamic details and more on champagne see:

• Sparkling Wine, Champagne & Co — Part 2,
  Klaus Roth,
  ChemViews Mag. 2010.
  DOI: 10.1002/chemv.201000043