I guess you forgot to read page 58 of the book cofty?
"By nothing, I do not mean nothing, but rather nothing -- in this case, the nothingness we normally call empty space. That is to say, if I take a region of space and get rid of everything within it -- dust, gas, people, and even the radiation passing through, namely absolutely everything within that region -- if the remaining empty space weighs something, then that would correspond to the existence of a cosmological term such as Einstein invented." [page 58]