Astronomy/A Pure-Energy Universe?
If all of the matter universe were converted into energy (or if it began as energy and never became matter), how much energy would that be? What could hypothetically be accomplished with a universe's worth of energy (assuming a civilization could contain and harness that much energy without being destroyed)?
Since the total mass of the Universe is only a fraction of the so-called "critical mass", converting all of it to energy would simply add to the existing Cosmic Background Radiation. It would have no significant effect on the expansion or future of the Universe, as it doesn't matter whether "being" is in the form of matter or energy -- either contributes to the "mass" equally, according to the formula E = m c-squared.
However, since you would like to know if that could be harnessed, I presume you don't actually want to presume everything is converted to energy -- just something close enough to your supposed civilization to take advantage of it. In that case, the energy from the conversion of even a single star into energy would be essentially unimaginable, and only the technology applied to harness and use it would impose any limits on what could be done.
As an example, even just the sunlight that falls on the Earth is many times larger than all the energy ever used by humans, and it is nothing compared to the energy that doesn't fall on the Earth and simply radiates into empty space -- and in the 10+ billion years of the Sun's lifetime, only about 0.7% of its mass is converted into energy, so you'd have to multiply all the lost energy by 10+ billion years times 140 (the reciprocal of 0.7%) to get the amount of energy available if we could just turn the Sun into energy. The result is a gargantuan amount, far greater than anything needed to accomplish wondrous things, or to destroy all life in the Solar System gazillions of times over.