Tally =?

Context:

This seemingly bizarre proposal is a consequence of the uncertainty principle of quantum theory, which says we can never be sure that a vacuum is truly empty. Instead, space is fizzing with "virtual" particles, which tend to disappear almost as soon as they form. In principle, however, they could stick around long enough to become real, if they could avoid adding energy to the universe's current tally - in accordance with the law of conservation of energy.