Just after the Big Bang, was the space, in its entirety, filled with Hot gas? After the Big bang occurred, was the space filled with hot gas totally?

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I'm not sure what you mean by "nebulae".  You could mean "matter in a gaseous or plasma state" or "cloud of low density gas and dust in space" (which is what astronomers mean by nebulae.)

The former case is believed to be correct, and it pretty well described in Wikipedia. I'll summarize. Immediately after the Big Bang (10-20 seconds or so) the universe was full of extremely hot and extremely dense matter -- hot enough that not only did atoms come apart, and nuclei come apart, but even protons and neutrons came apart into a quark-gluon plasma. This cooled into a stew of elementary particles, which cooled into mostly protons, neutrons and electrons, which cooled partly into nuclei. At this point we're maybe a minute into the expansion and the universe was full of a plasma that was still hotter than the center of a star. After around 300,000 years, the plasma cooled enough to recombined into atoms. Stars eventually started to form.

No astronomer would call any stage of this expansion a "nebula" -- they first formed later after the atomic gas left a milion years after the Big Bang started to condense into stars and patches of gas and patches close to vacuum.

Bottom line: Nebulae did not exist until millions of years after the Big Bang, but the universe was filled with hot gas much earlier in the Big Bang.