The definition of Big

Our personal space comprises of the space we occupy in every moment. Everything outside and apart from us is part of outer space.
The air and space around us, below and above us – it is all a minuscule part of an infinite vastness called the universe. So, how big exactly is our universe? Quite simply, it is the biggest thing we know. Yet, it is not as simple as that. It is beyond the comprehension of the human mind.

To give you a faint idea, take one light year. It is the distance that light travels in a vaccum in one year. Now, light travels at approximately 3,00,000 kilometers per second. Therefore, in a year, light would travel just under 10 trillion kilometers.

Now, as per the Big Bang cosmology, the observable universe consists of the galaxies and other matter that humans can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion. Simply put, everything that we are seeing today – the light from distant stars and galaxies – is from way back in the past, because we are looking at the light from the past which is reaching us right now. Some parts of the universe may simply be too far away for the light emitted from there at any moment since the Big Bang, to have had enough time to reach Earth at present, so these portions of the universe would currently lie outside the observable universe.

The age of the universe is about 13.75 billion years, but due to the expansion of space, we are observing objects that were originally much closer but are now considerably farther away than 13.75 billion light-years distance.

The diameter of the observable universe is estimated to be about 93 billion light years, putting the edge of the observable universe at a mind boggling 46–47 billion light-years away.
Take one light year. That is 10 trillion kilometers. Multiply it by 46-47 billion times. Light can only reach us from that far. Yes, our brain would explode if we could imagine. And this is just the observable portion. You get the drift.

Here is a cool video that puts things in perspective. Talking about perspective, a few months back, I came across this truly incredible model on the internet that shows us the scale of the universe. It will give you a better idea of everything mentioned above. If you click on every object given in this model, a short description will appear at the side. Zoom in, zoom out. Have fun.

Sources for this article: Wikipedia