Reality → Energy → Motion → Big bang
With the exception of few galaxies in the local group, the light spectra of all other galaxies show redshift [1] . In the late 1920s, Hubble discovered a direct proportionality between the recession velocity of galaxies (inferred from redshift) and their distances: the farther away the galaxy, the larger the shift towards red. The conventional explanation was that the universe is expanding and, consequently, must have come into existence with the big bang [2] . In 1964, Penzias and Wilson discovered the cosmic microwave background radiation, which became the strongest evidence for the big bang [3] . In 1998, evaluation of systematic new supernova measurements led to an unexpected new interpretation: the expansion of the universe is accelerating! [4] . As no known physical forces can explain the acceleration, the existence of ‘dark energy’ has been assumed [5] .
Light from stars and galaxies in a local group can be blueshifted, because at short distances galaxies may move towards each other due to the then dominating gravitational force (see also Cosmic motions, Note 3).
The big bang theory is now widely accepted because of corroboration from astrophysics (in particular nucleosynthesis as known from particle and quantum physics) and Einstein’s general relativity theory.
The cosmic microwave background radiation has been measured more precisely by two NASA spacecrafts, COBE (Cosmic Background Explorer), launched in 1989, and WMAP (Wilkinson Microwave Anisotropy Probe), launched in 2003. COBE measured an average black body radiation of 2.73 K temperature and provided a hint of regional fluctuations (averaging about 0.002%) that were measured and mapped in more detail by the higher resolution sensors of WMAP. The European Space Agency's Planck spacecraft, launched in 2009, confirmed the existence of these fluctuations with still higher resolution.
Spectra from near and remote supernovae 1a were compared to verify the then prevailing view that gravitation would slow down the expansion, but the opposite result was obtained: there seems to be an acceleration rather than a deceleration of the expansion. A more detailed evaluation of the data indicated that the expansion decelerated during the early stages of the universe's existence, but started to accelerate some 7 billion years ago.
The dark energy hypothesis led to the prevailing cosmological Lambda-CDM model (Lambda stands for Einstein’s cosmological constant and CDM for Cold Dark Matter). The overall mass-energy composition of the universe is now believed to be: 70% dark energy, 25% dark matter, and only 5% for all known 'visible' content of the universe, which comprises mass-energy of planets, stars, galaxies, gas and dust clouds, etc.