One simple way is to create a bottommost layer with a sufficiently large rectangle filled with black.
That is probably the best way except you have be alert to any transparency as it will show the background color through the over lying color.
The other way of making it a Live paint group but then that has pitfalls as well.
Thanks so much, again I am a newbie. I can make a new layer and then make a rectangle around all of my objects. Then I would assume that you could edit and fill with a color, but I do not see how to edit and fill with a color. Thanks again for the help. I have a strong understanding of PS and image processing. I am new to vector processing and am trying to learn as much as I can.
Once you have made the rectangle to serve as a background, and with it selected:
1) Click the large solid square at the bottom of the Toolbox (on mouseover, it should say Fill (Click to activate if not activated);
2) Click the little solid square underneath to the right (Fill) if not activated;
3) Select the colour in the Color palette/panel, using the desired values in the desried colour space (see the flyout for other options than the current one).
Obviously, you should have Stroke set to None (similarly chosen in the Toolbox).