  Although dark energy is a modern concept, some elements in it can be traced
back to the early part of the twentieth century. This paper examines the origin
of the idea of zero-point energy and in particular how it appeared in a
cosmological context in a hypothesis proposed by Walther Nernst in 1916. The
hypothesis of a zero-point vacuum energy attracted some attention in the 1920s,
but without attempts to relate it to the cosmological constant that was
discussed by Georges Lema\^itre in particular. Only in the late 1960s was it
recognized that there is a connection between the cosmological constant and the
quantum vacuum. As seen in retrospect, many of the steps that eventually led to
the insight of a kind of dark energy occurred isolated and uncoordinated.
