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as hubble looks out into these

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fields of galaxies we sometimes see

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clusters of galaxies these are galaxies

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that are held nearby each other by their

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mutual gravity

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these clusters are

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massive conglomerations

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there's so much mass that they have an

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actual observable impact on space time

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itself

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einstein predicted that mass distorts

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space but we didn't realize we could

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actually see the effects of that but

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with hubble we have been able to see

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distortions in space around clusters of

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galaxies the way we see that is when

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light from a background galaxy travels

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through that cluster of galaxies or

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around it due to this gravitational

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lensing effect the lensing also

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magnifies that background galaxy so if

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we look in some of these distorted arcs

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we can see more detail than we would

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ever have been able to see without

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gravitational lensing nature's boost

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so hubble has been used in an unexpected

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way for us to look at nature's

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magnifying glasses these gravitational

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lenses

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and using those to tell us about much

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more distant galaxies in detail we could

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never have seen without the lensing and

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also to tell us how dark matter is

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distributed in those clusters because it

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turns out that most of the mass that's

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distorting space and these clusters of

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galaxies

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is made of this unseen dark matter not

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the visible stars and the galaxies and

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we can't see the dark matter but by

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seeing how this background light is

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distorted we can kind of map out where

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that dark matter is through this effect

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we call gravitational lensing

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so we're learning a lot about dark

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matter this mystery of matter we don't

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really know what it is but we know where

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it is in these clusters because of this

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innovative use of the hubble space