Little is known about what dark matter and dark energy, the dominant components of the universe, really are. But the standard model of Big Bang cosmology, known as ΛCDM, incorporates how they outwardly behave. Dark energy, the model presumes, takes the form of a cosmological constant Λ, a constant energy density per unit volume of vacuum. Dark matter, meanwhile, is nonrelativistic (or cold; the CDM stands for “cold dark matter”), and it interacts with itself and with ordinary matter only via gravity and possibly the weak force.
With just a handful of free parameters, ΛCDM is appealing in its simplicity, and it generally agrees well with observations of the universe. But an exception is emerging in the Hubble constant H0, the universe’s present rate of expansion.

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