  We show that vector theories on cosmological scales are excellent candidates
for dark energy. We consider two different examples, both are theories with no
dimensional parameters nor potential terms, with natural initial conditions in
the early universe and the same number of free parameters as LCDM. The first
one exhibits scaling behaviour during radiation and a strong phantom phase
today, ending in a "big-freeze" singularity. This model provides the best fit
to date for the SNIa Gold dataset. The second theory we consider is standard
electromagnetism. We show that a temporal electromagnetic field on cosmological
scales generates an effective cosmological constant and that primordial
electromagnetic quantum fluctuations produced during electroweak scale
inflation could naturally explain, not only the presence of this field, but
also the measured value of the dark energy density. The theory is compatible
with all the local gravity tests, and is free from classical or quantum
instabilities. Thus, not only the true nature of dark energy could be
established without resorting to new physics, but also the value of the
cosmological constant would find a natural explanation in the context of
standard inflationary cosmology.
