.The short answer is you can't expand/collapse an entire Forum and view all its threads on one screen.
There are several types of messaging systems models in use today.
One is the listserv that uses an email engine to send all threads out to subscribers. The VAC is an example. These send out the threads in no order except you receive them as they are submitted. You have to maintain your mailbox to get any order in place. You receive all messages whether you have an interest or not.
Another example is the e-groups messaging system such as Yahoo uses. Messages are stacked as they are received and you have to click around to get any meaning to the thread. Again you have to wade through all the clutter to read what interests you.
This Forum uses a structured ordering system by topic, subject and then thread/replies. The program is VBulletin by Jelsoft. It is a PHP/MySQL server database system that allows for quick and accurate searches and cool features like the members section, who is online, photo posts, active posts, text editing features....
The only way to read all the threads is to drill your way down from the top. The best reason for this type of messaging model/structure is as the Forum grows (and this one is growing rapidly) it remains very orgainized, as the Admin/Moderators can work at keeping things structured by easily moving messages around to their proper topic locations.
Best Regards (and I know I am wordy);
-"You want to make it two inches - or, if you're working in centimeters, make sure it's enough centimeters for two inches."-Red Green