what are other semi-plausible theories besides the big bang?

A "theory" is a tool, a model that helps us understand how things might be working.

Most theories, once they are presented to other scientists, do not survive very long, as someone often finds evidence that contradicts it.

Some of them last a lot longer... and even turn out to be very useful tools, whether they are "true" or not. Newton's theory of gravity is now known to be "false" (or rather, not completely exact), yet it is still accurate enough to be used for everyday life... and its mathematical model (e.g., its equations) are far easier to use than the more precise theory of General Relativity.

The Big Bang theory started off rather simply (it was a simple, crude model). So far, no evidence has been found to contradict it. However, there are some things that are measured more accurately, they require us to add precision to the model, making things a bit more complicated, as we try to make the model closer to what is actually observed in the universe (for example, by adding to the model a period of "inflation").

Each time the model is made more precise, you could say that it is a new "variant" on the theory we call Big Bang.

So far, no other theory has explained the Cosmic Microwave Background AND other things that are also supporting evidence for the theory.

Explaining the CMB by itself (without explaining the rest) is relatively easy: it happened when the temperature of the universe (or, at least, the hydrogen it contained at the time) went from "above 4000 K" to "below 4000 K"

In fact, this is more solid that just a theory, because we can show in labs that hydrogen becomes transparent at (roughly) a temperature of 4000 K.

Hotter than that, the electrons cannot stay in orbit and a universe filled with free charged particles (loose electrons and loose protons) blocks all the light.

Cooler than that, hydrogen is transparent.

Somewhere in between (at the temperature of "recombination" of electrons and protons into neutral atoms), all the light is suddenly "liberated".

We know (again from labs) what the spectrum of light is at 4000 K (the peak is somewhere between yellow and orange, in the visible part of the spectrum).

Today, when we observe the CMB, we see the same spectrum... but it is shifted to lower wavelengths (because of the expansion of the universe) and the peak is in the radio portion called "microwave".

So far, so good.

The bigger problem is explaining why ALL the universe seemed to have been at that temperature, everywhere at the same time...

The only other serious theory (before the discovery of the CMB in 1964) was called Steady State, which stated (in its model) that the temperature of the universe does NOT change over an eternity (towards the past or the future). That is why the discovery of the CMB killed that theory.

All currently viable theories of cosmology are variants of the Big Bang theory.

Lambda cold dark matter big bang is the current "standard theory".

The Big Bang is essentially observed. It's no more likely to be wrong than the "heliocentric theory of the solar system".

The so-called CMB is simply the thermal radiation of the molecular hydrogen that fills interstellar space. In other words, it is sourced from within the Milky Way and is not a background to the galaxies at all. This is evidenced by the following observations:

Galaxy clusters do not create shadows on it:

http://www.moondaily.com/reports/Big_Bang_Afterglo...

Cool spots are not lensed by galaxy clusters:

http://www.spaceref.com/news/viewpr.html?pid=17752

CMB aniotropies match the distribution of atomic hydrogen in our local stellar neighbourhood:

http://www.spacedaily.com/reports/Cosmological_Dat...

The CMB's polar signatures are aligned with our solar system and it's motion through the Milky Way:

http://www-personal.umich.edu/~huterer/PRESS/CMB_H...

http://www.phys.cwru.edu/projects/mpvectors/

http://www.astronomy.com/news-observing/news/2004/...

Btw, this molecular hydrogen accounts for the missing mass that has been given the name dark matter: http://web.archive.org/web/20100702194353/http://h...