Is the Big Bang theory incomplete? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

OK, I’ll spare you another whimsical remark about The Big Bang Theory being a television show (obviously incomplete as more episodes are still being produced) but I’d like to point out that the Big Bang is not a theory. Rather, it is a phrase, originally an expression of ridicule, used by the astronomer Fred Hoyle in a 1949 BBC radio show, who opposed to the idea of an expanding cosmos.

The theory that predicts a cosmos that is either expanding or contracting but never static is Einstein’s general relativity. (Einstein perceived this as a flaw, and attempted to fix the theory by introducing the so-called cosmological constant.)

The expansion can also be observed, as demonstrated first by Georges Lemaître, and then by Edwin Hubble and others.

So the theory of general relativity predicts that our cosmos is expanding (or contracting, but that contradicts observation, so we pick the expanding version.) Is this theory complete? Not by any means. General relativity is a theory of the geometry of spacetime. It is not a theory of matter. Matter is simply represented by the so-called stress-energy-momentum tensor; the nature of matter is unspecified.

Modern physical cosmology combines several theories to construct what is sometimes called the “concordance” or “Lambda-CDM” model of the universe. In addition to general relativity, the Standard Model of particle physics is used (which, in turn, is constructed using quantum field theory, along with specific postulates that describe the known fields and particles) in the construction. The model has been very successful describing, and in some cases predicting, (before actual observations were made) detailed characteristics of the universe.

In addition to the known bits of physics that I mentioned, the concordance model also invokes a big unknown: collisionless cold dark matter (CDM), which is supposed to have an average mass-energy density that is about six times that of “normal” matter. The nature of this CDM is completely unspecified, other than the fact that it is a) cold - so its thermal kinetic energy is negligible compared to its rest mass and b) collisionless - no pressure, no interaction with itself and no interaction with “normal” matter. What this CDM is, we have no idea.

Furthermore, the model incorporates a cosmological constant (denoted by the Greek capital letter Lambda), which is permitted by Einstein’s theory, but what it actually represents is unknown. Is this just a constant of Nature? Is it a stand-in for something else, such as an unknown field, or perhaps the residual vacuum energy of quantum fields? Is it something else? No convincing answer is known.

The concordance model accounts quite well for the observed matter content of the universe, including the ratio of various primordial isotopes. It does not, however, convincingly explain why there is an imbalance between matter and antimatter. There are also other mysteries, such as the extreme smoothness (homogeneity) of the early universe, which led to proposals such as cosmic inflation, but these proposals often raise more questions than they purportedly answer, and in any case, they are not supported by observational evidence.

Lastly, when we look at the extreme early universe, when the quantum effects of gravity can no longer be ignored, we run into a fundamental theoretical difficulty: The fact that we lack a quantum theory of gravity (indeed, some authors even speculate that gravity may not be quantized at all).

This question originally appeared on Quora. More questions on Quora:

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