Both popular science articles and many textbooks describe the Casimir effect — an attractive force between two closely spaced neutral conductors — as being due to the exclusion of some of the zero-point modes of the electromagnetic field, whose wavelengths are greater than the separation between the plates. In this account, the vacuum between the plates has less energy than ordinary vacuum.
A formula based on this model seems to agree reasonably well with the measured result. This has encouraged people to proclaim that “vacuum energy is real” and (sometimes) to suppose that this model can be pushed much further to predict larger regions of greater negative energy.
But in fact this force can be modelled as arising between the plates in a far more conventional way, related to van der Waals forces, which makes no reference at all to “vacuum energy”, and the formula based on zero-point modes can be seen as merely approximating a more accurate formula based on the material properties of the plates.
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