Nothing spreads faster than rumour. Enterprising wizards noticed this and began conducting experiments. This is how whisperspace was discovered.
It's not just that rumours travel as quickly and widely as people do and so unless you're first the rumour is most likely already there, whispered things actually travel in their own dimension. A critical mass of rumour-spreading or secret-sharing leads to a story breaking through into whisperspace from which point it can wind up nearly anywhere that stories are being swapped.
The whispered stories might slip into the head of a struggling playwright, or pop into the mind of a bombastic storyteller in a bar who was running out of new stories to tell, or be half-remembered by the old lady who sells necklaces on the street as something her son told her, or was it the local drunk, or a sailor perhaps? Either way, they can end up places they have no right to be.
When things begin to be seen as true, or old hat, they are ejected from whisperspace. This would typically be entirely unnoticeable. When wizards started messing with whisperspace, however, it became relevant. It placed a hard limit on how long you could keep something in whisperspace, and on what kinds of things you could send through it.
Very few wizards study whisperspace. Many wizards make use of it, however, as certain spells that have entered into common wizarding knowledge (eg Message, Suggestion) take advantage of whisperspace. The key is that only a few know these spells do so. If everyone knew, they very well might not work.
In fact, those wizards that study whisperspace also do their very best to spread misinformation and doubt about it and dissuade others investigating it. The concern is that if whisperspace itself became common knowledge the whole incredibly useful system would come undone.
Suruscant the Sculptor was the first notable whisper wizard, as notable as they get which is typically not very notable once they work out how to remove things from whisperspace. His specialty was shaping stone. For years he lived a druidic lifestyle and shaped stone sculptures that whispered as the wind passed through them. He meditated in small forests of the sculptures to learn the secrets of the land. (A nature-oriented magic-user such as a druid can meditate in one of these sculpture forests for an hour to find the answer to one of the following questions: Where is clean water? Where is food? Where is safety? Where is danger? The answer will be from the perspective of the animal and plant spirits in the surrounding area, so might not necessarily align with the askers ideas of good answers to those questions)
He began to follow the whispers and started to hear the whispers of not just the earth and the trees and the beasts, but of other people. He followed them and the whispers got louder and louder. He eventually came to a city where, over the course of many years, he shaped a smooth stone chamber underneath it. Hundreds and hundreds of tunnels of all sizes lead out from the chamber, their exits are in hidden places all over the city. The tunnels amplify the city’s whispers and carry them to the chamber (spend ten minutes here for a random piece of gossip, it mostly sounds like white noise so you might be better off in a pub but there are some things that aren't whispered in pubs that wind up in the whisper chamber).
It was here that he began to study and learn about whisperspace. He documented ways to send objects into whisperspace and keep them there, noting the side effect that people would start thinking about objects stored this way more often without knowing why. His writings are highly sought after, but very difficult to find. It is rumoured that they span only a few weeks, after which nothing more is left of Suruscant except his whisperchamber.
Some think that he removes everything he does from whisperspace and that is why there is no more information.
"But then why do we still whisper about him?" Others whisper, "Clearly he must be in whisperspace and that is why we cannot find him."
Getting into Whisperspace
No one really knows how to get a person into whisperspace. There are plenty of rumours, of course.
Whisperstone - Small foggy white gem. When rubbed will capture up to one minute of whispered conversation within 60ft. Rub again to release the whispers (makes a noise like running a wet finger round a wine glass which morphs into voices)
Whisperspace is a vast desert with a raging tempest of thoughts swirling around it constantly. A strong gust of wind could knock out some or all of your thoughts and memories, replacing them with new ones. Thoughts that you lose could end up popping into anyones head. Its a good idea to have some kind of psychic protection or you might soon become a random collection of misheard stories, scandals, and secrets.
You can make a will save to prevent loss of thought as well, but that will only hold out so long. Wizards and other magic-users can easily lose their memorised spells this way.
There is a kind of a sun (it's heavily obscured and dull) that travels east to west (analagous to east to west in the real world). The scale of distance (and time) is all messed up and inconsistent, however, so a couple steps might equate to a few meters or a few kilometres depending on where you are going and in what direction you're travelling. If you stop and listen to the whispers for a while you might pick out something that gives you a clue as to where you are relative to the real world, but the wind carries whispers all over so it's not a guaranteed method.
You are ejected from whisperspace when you think clearly of, or explicitly refer to, yourself. A mirror is an easy way to achieve this.
The collectors are tall robed figures with spindly stilt-like legs. Their heads are all shaped differently, but they look similar to the sculptures of Suruscant and all interfere and interact with the thought wind in different ways. Apart from their contours and the holes that reverberate with the wind, their heads are completely featureless.
They are nomadic. They follow the type of secret they collect. Once they catch up to a swirling vortex that contains some ideas they are interested in they will sit and begin rapidly scrawling them out on paper, angling their head this way and that to catch the wind just right.
They also carry around some junk secrets they picked up that they aren't interested in collecting. They will trade these with other collectors, or whoever comes along, for secrets more to their liking.
When a collector dies (they don't put up much of a fight), their collection is swept up into the tempest. If you kill a collector, some people somewhere will spread a rumour about you. The rumour pops into your head straight away, so you know what it is. It oftentimes has more relation to their collection than to the truth.
The sand in whisperspace can be used to make whisperglass. When whisperglass is shattered, those who hear it will blurt out an uncomfortable secret. If the person has no uncomfortable secrets they will lose their voice for 6 seconds (or until everyone has finished telling their secret, which is usually within that timeframe since they aren't required to give lots of detail).
Some Collectors have whisperglass jewellery. Carrying whisperglass makes it a lot easier to stay in whisperspace as it mitigates the winds that might carry whispers of you off into someones head and pull you out.
Hearing whisperglass break in whisperspace also pulls you out as it forces you to consider yourself.
You know all those stories you hear of terrible monsters, bogeymen, indescribable horrors that lurk on the edges of our lives? Fortunately, most of these things are merely rumoured to exist. Which, unfortunately, means they do exist in whisperspace. When Suruscant first found a way to travel into whisperspace he immediately realised his mistake. Him getting in, meant other things could get out. He had not planned on staying in whisperspace forever. But now, he must. He must maintain his prison and ensure that none of the things people tell whispered stories about become more than just stories.
With his powerful magics he hunted down these monsters and trapped them in a vast whisperglass prison of his own creation. If he dies, they will no doubt escape.
The one rumour Suruscant could not imprison was Death. Death that whispered of being that noone has ever seen but which takes lives and harvests souls. Yes, Death lives in whisperspace. In reality, Death eats the souls of everyone who dies. Death is skilled at navigating whisperspace and at moving between whisperspace and reality, which is how they can visit every single person just as they die.
If you meet Suruscant he would certainly appreciate you capturing Death, though he does not believe it possible.
If Death is prevented from leaving whisperspace, or from returning to it, souls will start going to the proper places and ghosts will be a bit more relaxed (they didn't have to slip away from a terrifying multidimensional being just after they died to become one).
So, that's whisperspace, don't tell too many people about it.