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There is no denying that there is good and bad in this world.  We live in the gray area, depending on the choices we make.  It is sometimes said that one cannot be without the other; that there is always some light in darkness and always some darkness in the light, but that is pushing it to far.  There cannot be any darkness in the source of all Light. Try looking in a lamp you'll know what I mean. Darkness per definition however, is caused by a certain lack of light. In free space, or better yet; "perfect" vacuum, light can move on endlessly, but as soon as matter gets in the way, a shadow is thrown.  

It is fair to say that it's hard for light to penetrate matter, from the outside in, but also from the inside out.

Since our bodies are made of matter, it is said that we, that is our light-souls, are living in darkness, or as the Bible literally would say; we are dwelling in the shadow of death which also stands for the material world.  

Of course it is a different kind of light; a different kind of energy or spirit, which is an other word for energy, but it apparently works out the same.  If you face a fireplace, your front will be warmer than your back, because your back is in the shadow of your body.  Plus the warmth will only penetrate so deep. Only by longer exposure to the heat your whole body will warm up.  In the same way our material body darkens the light of our soul that dwells in it.

Throughout the whole cosmos there is detected a form of energy, (light and dark) that appears as a static noise in the headphones of observers.  The cosmos is filled with matter in the shape of clouds of gas and dust, rocks, planets, suns, recently discovered “Dark Matter”,  and what else have you.  Even in Black holes, there is light shining, only it is pulled towards the center which is why we cannot see it.  On the edge of the cosmos where light hasn't reached yet, of coarse it's dark; it's the outer darkness. If it all sounds very simple, it's because it is, Some people complicate it by suggesting that there was some god of darkness.

Time began when the Light was turned on; “Let there be Light.”  Ah, you might say, but before that there was darkness without light.  Maybe so, but the essence of this creational Light is the Life which sets things in motion.  Before this, there was no Light, no life, no thought, no evil, no god of darkness, “no nothing”, just an empty dimension, and probably not even that. It all started when God manifested himself while creating this dimension.

Give ear to me, Jacob, and Israel, my loved one; I am he, I am the first and I am the last. Isa 48:12 

In the beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God.  

All things were made by him; and without him was not any thing made that was made. John 1;1

The Prakriti (nature; matter) is My lower energy. 

My other higher energy is the Purusha by which this entire universe is sustained. 

Know that all creatures have evolved from this twofold energy, and Brahman is the origin as well as the dissolution of the entire universe. 

Bhagavad-Gita 7.05

I am the First and the Last, the start and the end.  Re 22:13 

So, we can assume that before the Big Bang there was a hidden God, who would start it all, but there certainly was no god of darkness, no Satan, because other than God Himself, there was nothing at all.  Not in this universe there wasn’t. 

Darkness is a lack of something; not a presence of something.

If there's nothing there, there's nothing there.

This something is called Light, which is energy which in old days, people used to call spirit. First there is the Light and the light brings life. Time it is said, started with the Big Bang; the unleashing of creational energy, in other words; Light; Spirit, the willpower of God.

In a spiritual sense matter; our body darkens the Light that is within all of us. Some people are reported to have like halo's around them. Some people, when the enter a room the whole room seems to lighten up. This inner Light has to be amplified to be noticeable. Material needs and urges of the body literally block the spiritual Light that is within us.

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The Outer Darkness on the other hand, obviously, is the shadow thrown by Matter. The darkness around the campfire, or the nothingness surrounding creation.

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Some scientists like to point at the fact that this Cosmos is going to fade away into darkness after having expanded so much that all matter; all galaxies, suns and planets will disintegrate, as if to say; See there is no eternal life, everything is going to waste. -The present world, however is neither the first nor the only world in existence. Before the world was created there existed already many others. God created many worlds and destroyed them until He created this world and spake, The other worlds did not please me but this one does please me. (Genesis Rab. 3,9)  The pre-existing worlds were preparations for the present one. In this mythical conception lies the idea of an organic progress. When the pre-existing worlds had fulfilled their purpose they disappeared. Not only the worlds, however, but also their inhabitants had been created and destroyed; they came and went. Nine hundred and seventy-four generations had existed before the creation of this world, but they were swept away because they were wicked and did not please the Lord of the Universe. The idea is that the souls of the persistently wicked, who remain under judgement (Reincarnation ) are on their way of being destroyed along with the universe while falling apart in the outer darkness. Of coarse they will be gone long before that. Some souls are spirits sent down upon earth and ordered to inhabit a human body as a punishment (according to their Karma) for faults committed (in earlier lives). For others it is a test and an opportunity to show their strength. Angelo S. Rappoport, Ancient Israel p.9, 21

Maybe indeed if we keep it up like this this Cosmos is going to waste but our souls may survive to live another day on a new Earth under a new heaven.

“See, I will create    new heavens and a new earth. The former things will not be remembered, nor will they come to mind". Isaiah 65;17