Quote:
My understanding is that there is no evidence that it exists.


There have been experiments such as this:

G. Bressi, G. Carugno, R. Onofrio, G. Ruoso (2002), "Measurement of the Casimir force between parallel metallic surfaces", Phys. Rev. Lett. 88 (2002) 041804.
(http://www.arxiv.org/abs/quant-ph/0203002)

Also, proposed setups to testify, like this:

Davis, E.W., and Puthoff, H.E., "Experimental Concepts for Generating Negative Energy in the Laboratory," in these proceedings of Space Technology and Applications International Forum (STAIF-2007), edited by M. S. El-Genk, American Institute of Physics, Melville, New York, 2007.