If the universe is expanding, how is it possible that galaxies can collide with each other? It turns out that not only are colliding galaxies possible in an expanding universe, the fact that they occur is due in part to dark matter and dark energy.
The Shape of Things
The most widely accepted model for the universe, sometimes called the standard model of cosmology, is known as the ΛCDM model. The Λ or “lambda” refers to the dark energy parameter known as the cosmological constant (which often uses lambda to represent its value). The CDM stands for Cold Dark Matter, which is the type of dark matter currently best supported by observation. In the ΛCDM model, the matter of the universe consists of regular matter (about 5%), cold dark matter (about 27%). Dark energy is then caused by a cosmological constant, which is a property of space and time itself, giving rise to dark energy.
Bend and Stretch
Since space itself is expanding, the wavelength of the traveling light also stretches. This means that while the light travels, it continues to redshift due to cosmic expansion.
Ripples on the Cosmic Pond
The amazing thing is that all these values fit in this single curve. If the values were different the peaks would shift left or right, or be higher or lower. While the image of the CMB is wonderful, with its swirls of color, this graph is even more wonderful. It tells us that our understanding of the universe is on track.