In the late 90s, measurements of the luminosity distance of type Ia
supernovae have shown evidence that the Universe expansion is
accelerating. This result came as a great surprise, since all known
forms of matter lead to a decelerated expansion. The possible
explanations of this phenomenon can be divided into three classes:
the vacuum of our Universe has a tiny nonvanishing energy (a
cosmological constant); some material with extremely unusual
properties (quintessence) is responsible for the acceleration; or the
behavior of gravity at ultra-large distances is different from what
we expect. I will discuss the different aspects of such explanations,
paying special attention to the point of view (and the prejudices) of
particle physicists.