General relativity explained a mechanism to contract this huge distance into few meters. Albert Einstein called this mechanism ‘bridges’ in space-time. Today, scientists call them wormholes. A wormhole would act as a shortcut connecting two distant regions in the universe.
When you reach that galaxy today and look back at Earth, you would not see Earth as it is today; instead you would see Earth as it was 100 million years ago (you would see the dinosaurs). This is because light took 100 million years to reach there. So as you use a wormhole, events at Earth (your departure) would appear as if in fast rewind (from today till 100 million years ago).
On the return journey, events on Earth (destination) would appear as if in fast forward (from 100 million years ago till today); while the events at that galaxy (departure) would appear as if in fast rewind (from today till 100 million years ago). This is not time travel; this is just a consequence of the length contraction inside a wormhole.
Physicists know how a wormhole works but have never actually used one. Do you remember how a frictionless roller coaster works? That is, you start high and slow then you go down and fast but when you come back up again to your previous height you return to your previous slow speed? Well, the wormhole is the most efficient roller coaster you can imagine. Energy is not required to move through the wormhole, that is, you can simply switch off your rockets. Gravity accelerates you and pulls you through and then expels you the other side. You will feel like being swept by waves at the beach; nothing more.
On your way through, gravity causes your clock to run slower and your ruler to shrink. Distant observers see you accelerating to relativistic speeds; as a result, your energy (and mass) increase dramatically in their view; but when you get out the other side everything returns back to normal (your clock, ruler…).
God gave this method of transportation to his angels throughout the universe. The Quran calls them ‘Ma’arej معارج’ and describes how angels use them for long distance travel. Today Moslems know that these ‘Ma’arej’ is what scientists call wormholes:
[Quran 70.1-4] Someone asked about the penalty that will befall 2 the unbelievers; (A penalty) that has none to ward off; 3 (a penalty) from Allah (who owns) wormholes [Ma'arej in Arabic] 4 The angels and the Spirit ascend to Him in a day, the measure of which is fifty thousand years.
Any object passing through this wormhole will also experience this same time dilation (one day inside wormhole vs. 50,000 years on Earth) and not just those angels. This time dilation does not say how long it takes you to cross the wormhole, it just says that when you pass through the wormhole you age by this ratio. This space is called hyperspace (برزخ Quran 23.100, Quran 30.55-56).
God says that those wormholes are doors in the heaven that contract interstellar distances into walking distances; the resulting scenery is bizarre: God says that those who do not believe in the message will not believe it even if He showed them a bigger sign. He created beautiful towering structures in the heavens. If He opens for those nonbelievers a door in heaven and lets them continue passing through it to those distant structures, they would not believe that they really got there by simply walking few steps. Instead they would think that they are just optical illusions:
[Quran 15.13-16] They do not believe the Message, like those who preceded them; 14 Even if We [Allah] opened upon them from the heaven a door and they continued passing through it 15 they would say ‘Our sight is bedazzled, rather we have been bewitched’ 16 It is We [Allah] who have made towering structures in the heavens and made them beautiful for beholders. 17 And We protected them from every evil spirit accursed.
Here, they wouldn’t believe their own eyes thinking that what they see is not real. They wouldn’t believe that they got to those heavenly structures by simply walking few steps. But God insists that what they see is real and not illusions (that is, they really got there).
The Quran says that those heavenly doors are galactic shortcuts to distant places in the universe. Angels guard those heavenly doors ‘from every evil spirit accursed’. Moslems believe that angels use these doors for long distance travel. Angels can accelerate up to the speed of light for domestic travel; but they use these wormholes to reach any place in the universe before you finish reading this sentence.
Moslems also believe that wormholes are not strictly for the use of angels. Their prophet used a wormhole once in the ‘Israa & Me’raj’ (Me’raj معراج is singular of Ma’arej).
[Quran 34.6] And those who have received knowledge (scientists) will see that what have been sent down to you from your Lord (the Quran) is the Truth, and that it guides to the Path of the Exalted, Worthy of all praise.