Debunking dice conspiracy theories
There are many ways to debunk the various conspiracy theories about dice on online backgammon servers..
The simplest is perhaps to ask 'Why (would they)?'
First you can look at cases in history where something was rigged. Perhaps most common would be rigging some sports events (boxing or other) so that the outcome is known upfront. This would allow someone to place a sizeable bet knowing that the outcome is guaranteed. So the motivation would be winning a lot of money.
Now back to online backgammon servers. For someone to be tempted to actually take the risk of their own reputation and current revenue assuming they are flexible with doing right or wrong to begin with the gain would have to be quite sizeable. $1 million dollars? And that would be for the site owner and someone else placing some serious bets standing to gain even more. Have you often heard about such bets being placed on the winning of a single backgammon match?
The ones who complain about the dice are unlikely to have played a match where even $1000 was at stake. More likely just a few rating points and you think a site would be willing to risk their livelyhood on that? Robbing you of a few rating points for what purpose?
So if it isn't any money at stake you would have to come up with an argument that the site owners take a sadistic pleasure in messing with your head giving you an endless stream of rubbish dice. In that case you are confusing the game of backgammon which all on its own is able to do this and for that reason has gained the reputation of being the 'Cruelest game'.
There is no such thing as playing for money on Heroes so it makes no difference to the server who wins a match so there is no 'Why' to be found there either.
Another possibility for 'Why' would be to give premium players better dice so they stay happy and keep playing on the site and keep subscribing. The odd thing is that premium players also complain about rigged dice so this doesn't add up to a meaningful argument either.
Understanding the dice/randomness to begin with
Understanding what dice/randomness is to begin with without ever having studied the subject at a sufficiently high level (typically University level) is perhaps a feat that is out of reach for pretty much everyone. Even if you have looked at or played backgammon for your entire life you may not be able to write down a single formula that can be used to describe randomness.
Can you hold 1 million numbers in your head and then tell how many occurences there are of each numbers against the expected occurences based on the theoretical chance of a number to appear? No you cannot. Perhaps you have over time managed to create a magical skill of detecting randomness? Which doesn't involve any actual math or statistics? Is 55 55 55 55 55 55 random?
Start your education now and learn about the magic of randomness: Video about randomness. From this you may start to get an idea that you are confusing experimental randomness (observed randomness) against theoretical randomness. They are not the same. The first approaches the second over a long time.
Become better at backgammon
Players who complain about the dice are usually not very strong players to begin with. There could be something here which is closer to what is going on and why you seem to be getting so bad dice. If you play your move in such a way that you increase your own chances of doing something good with your next roll then on the surface it seems that you are getting luckier when you start getting more rolls that you can make good use of.
Becoming a strong backgammon player can take a lot of time and it may not be easy to spot any improvement other than you start getting better Performance Rating results. Knowing that at least you played as well as you could should be a small consolation even if the dice gods decided that the opponent was the one who should end up winning the match.
Invest in a few backgammon books and start improving your game!
Some sites allow you to practice your skills like Backgammon Studio and the next generation Backgammon Studio 2.