[amalia (OP)]

Eddie asked us on Twitter:
Why do cosmologists say the universe should have more mass? If the Big Bang was an explosion, the majority of its matter is in the shell. As this shell expands in space, why couldn't 90% of its matter have ended up far beyond where we can see, 100 billion light years away?
Do you know the answer?

[Petrochemicals]

Its not more matter total that they are looking at,  its density of matter per area, galaxys rotate too fast for the matter therein to be contained under there own gravity, they should fly off in pieces if only under the gravitaional attraction of the arrangement  of the mass within the galaxy.

[PmbPhy]

Eddie asked us on Twitter:
Why do cosmologists say the universe should have more mass? If the Big Bang was an explosion, the majority of its matter is in the shell. As this shell expands in space, why couldn't 90% of its matter have ended up far beyond where we can see, 100 billion light years away?
Do you know the answer?

Astrophysicists say that, not cosmologists. And they don't really say that. They are referring to what they call dark matter. They postulate its existence from what they observe from the rotation curves of galaxies in order to explain why the curve is what it is. I.e. outer stars are revolving about the galactic center at a rate different than what Newton's laws predict. They move faster than they "should."

[Colin2B]

@PmbPhy
What about matter beyond our visible horizon? I assume that would exist, but have never looked for any detailed studies.

[Bill S]

Marcus Chown.,  (Our Special Place.  New Scientist, 17.11.2008) looks at the possibility that we might inhabit a “cosmic bubble” in which average density is lower than in the rest of the Universe.
 
He describes how this cosmic bubble could explain the apparent acceleration of the Universe’s expansion, without the need to invent some mysterious dark force.

 “In such a low density region, the breaking pull of gravity is weaker, and so the region would quite naturally be expanding faster than the more dense area enveloping it.  A bubble surrounding us, covering the volume from which light emitted over the past few billion years is just reaching us, would be just the thing to explain the supernova observations.  Observing from within the bubble, but using distant supernovae as yardsticks, we would see a universe whose expansion seems to be occurring faster than it used to – without the need to invoke dark energy.” 

I suspect that, in order to obtain the values we observe for the cosmic background radiation, we would have to be in the centre of the bubble, or extremely close to it.  What are the odds on that?

Oh, wait! This was in New Scientist!