ic5 antimatter dimensions
IC5 done quick

First, you may want to test a few things yourself and see why it is hard to get many 8th dimensions (buying dim 5-7 is especially tricky).

If you just want the trick to get galaxy 21 fast without reading the guide, it is to buy lots of dim 5-7 by jumping their price over the price of dim 8 at the beginning of an infinity, with some dim 1 buys.

Now all the details :

A quick remark is that buying a single dimension unlocks the next one but does not affect any price, so you can ulock dim 8 without raising it's price. A second thing is that nothing modify the price of tickspeed, and pressing T can't affect prices you can't afford, plus collision with prices above 1e500 is extremely rare due to huge gaps between prices.

Phase 1 : getting to 20 galaxy
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You have some autobuyers settings to do:
dim boosts -> 0/more than 22/1              The goal is to just do the dim shifts. the bulk buy should not be too high 
                                            or else dim shifts won't be done for whatever reason
galaxies   -> more than 21
big crunch -> whatever
dim 1-7    -> buy singles                   To avoid raising cost of dim 8
dim 8      -> buy max, priority 1           This puts the price of dim 8 very high
tickspeed  -> buy max, priority 2           This buy normally doesn't affect dim 8 because the prices are lower
sacrifice  -> x2 for example
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Start the challenge and go to the toilet (1 min)
Tt should be stuck to 19 galaxy, click the dim boost and tada you are at 20 galaxy and ready to get quite a few boosts.

Phase 2 : get this galaxy 21

Buy a single of dim 1-7 and 10 dim 8, so it's cost should be 1e40. Then keep buying dim 1 until the costs from dim 5-7 jump over 1e40. Now you can hold 7, then 6, then 5, and you just gained a lot of multipliers! Then hold 8 (and from now on buy every dim 8), and now you just have to figure out how to manage dim 1-4. 
There is a perfect buy strategy which is very annoying, if you are lazy maybe holding 1 is good enough, and a quite good strategy is to alternate 121212, it buys every dim 1 and dim 2 possible. When you can't buy anything press T, and buy dim 8 and mash 121212 and press T, it should get you the galaxy 21 easily.

Phase 3 : get 1e11111 antimatter
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First unlock every dimension (by pressing 12344 123455 1234566 12345677).
Then buy single 1-7 and lots of dim 8 (you can press T) and dim boost.
If you really want to speedrun (under 10min) you can redo a strategy like phase 2 to get more dim boosts, or else do the final run where you don't care about buying lots of dim 8 :
Buy a single 1-7, hold 8, hold 7, hold 6, hold 5, mash 121212, press T when you can't afford anything, continue buying everything you can, and maybe wait a few minutes and it's done.
ic5 antimatter dimensions