The answer is in httpd.conf in the /usr/local/apache/conf directory on .tar.gz installed apache. On an rpm install I wouldn't know, I haven't run the rpm version of apache in many years.
#
DirectoryIndex: Name of the file or files to use as a pre-written HTML
directory index. Separate multiple entries with spaces.
#
<IfModule mod_dir.c>
DirectoryIndex index.html
</IfModule>