Once I met young man, acute psychic and energy healer. He was clearing human energy fields from dark entitties or "monsters', as once he called them.
He also explained the nature of that monsters. "Every imperfect thought is already a monster, so we emit them every minute", he said.
His statement left profound impression on me. What is imperfect thought, then? We could say that it is a though that lacks love.

There also can be more profound explanation. Once I draw a diagramm showing male female polarities. Unfortunately, can't upload it here still.
On that diagram were shown 4 polarities.

Spiritual Male polarity were opposed by Material Female. Spiritual Male represented connection to source and spiritual world, but also - abstinence, rejection of possesion and physical pleasures. It is opposed by Material Female - which represents greed, desire to collect and posses, dependence on material pleasures. If these two polarities reject each other, your life and relationships are imbalanced. When you find center between these polarities, you are connected to Source, but also can enjoy material pleasures, can give more and also receive more. In the case when these opposites are "married" you feel that there is always enough for you. You see in our world these pair of opposites are out of balance.

The other pair of opposites - Spiritual Female and Material Male. Spiritual Female represents mercy, compassion, absolute understanding and self-sacrifice. Material Male is aggression, personal power, leadership. The return of the Goddess( that is widely spoken) is return of Spiritual Female, as far as all other components in our world are richly represented. If these opposites are married, we are full of love, but are never abused or feel victimized. We can assert our Truth and calmly defend ourselves. There is also peace and understanding in our hearts.

Our conscious should dwell between these spiritual-material, male-female opposites. If it is tilted towards any polarity, then the though is imperfect.Then the "darkness" is created. The more it is tilted, the more is imperfection. Such imperfection creates challenges in our lives and relationships, because we emit "entities" towards our own or other people's energy fields.

If you find which imbalance is represented in your life, then you should focus on opposite polarity to develop and express it in your life.
If such dark entity or "imperfect thought" from other human being dwells in your energy field, it means that you also have imbalance of opposites in yourself, otherwise it couldn't survive in your aura. You are also "out of centre" in the same realm of life.That's why it is widely stated tht we shouldn't blame others for our misfortunes. To overcome such "dark enetity" you just have to restore balance in yourself.

And then the darkness is replaced by light.