The answer

What do most physicists believe the origin of the universe, or first cause, is?

First of all, I would like to say that if you want more information or need clarification, get it yourself. Wikipedia is an infinitely useful scientific research tool, even though the liberal-artsies hate it for some reason. Alan Guth also has a good, technical but math-light review article.

Assuming we have some familiarity with the Big Bang and evolution, most of the universe seems to work like clockwork if you take it back to 10^-34 seconds after the first explosion of a lot of mass-energy that was the Big Bang, which essentially started in an infinitesimal point, a singularity (which is a fancy word for our ignorance which will be resolved with a theory of quantum gravity).

Where does all that energy come from - what is the first cause, and how do we get something out of nothing? The simple first cause is quantum mechanical fluctuations in the vacuum. Energy can be spontaneously created from nothing, as can mass, as long as it's destroyed quickly. This is a theoretical and observational fact of quantum mechanics.

Quantum mechanics is weird stuff, I know, but if you don't know it and don't care to, all I ask is that you believe it. Believe it because it's the reason why MRI, electron microscopes, and lasers all work, and why we know mathematically how they work to a precision equivalent to knowing the distance from New York to Los Angeles with less than a human-hair's width of error (yes, this is all quantified).

So mass-energy gets created, as does the stuff to destroy it, in the middle of a perfectly symmetrical unscaled pre-universe which we can call the true vacuum, or the simplest state possible (It is only now that we need even consider the specifics of 4D spacetime - up to now all that we care about is symmetry, which ensures our math to work). This mass-energy is no longer vacuum, and so usually it's unstable and destroys itself quickly. But sometimes it's enough to climb into a new kind of stability called a false vacuum, a metastable state, where it can rest easily and not get destroyed. Here's the real kick - this mass-energy can be self-repulsive and actually expand out in the true vacuum, where it can spontaneously (because of quantum mechanics) have pieces break off, either collapsing into the true vacuum or some other, more stable false vacuum. These pieces are trapped inside this false vacuum bubble, and the break-off moment releases lots of energy, up to as much as was used to create this mega-universe in the first place. BANG!

This scenario is quite nice, in that it explains a lot more about the universe than what I've mentioned here (see the WP article on cosmic inflation). It requires two theories: quantum field theory and general relativity, the former of which is the most accurate description of nature ever, and the latter is widely accepted with only some points contested on the largest and smallest scales. One consequence is that it opens the door to multiple universes with differing fundamental constants of nature. It has many flaws, however, which are not addressed and likely won't be addressed until the particles involved in this theory are sorted out and we understand quantum gravity. Another bad point is that since we're in a metastable state, there could be a true vacuum seed that wipes out the entire universe at the speed of light, the ultimate catastrophe. Whatever, you won't feel a thing.

To summarize: quantum mechanics allows for something out of nothing, which can find itself a stable home and with general relativistic theories of gravity will expand out and form Big Bang universes.